Why are you even asking that question since the way you phrased it already betrays you personal bias with terms like "miserly"?
There are obviously 2 often polarizedly opposed camps on this issue. Does one side have to be right and the other wrong? Or is this a case of what seems right for one person is not necessarily right for someone else? Or, as you yourself suggested, is it a matter of different approaches being appropriate depending on the particular circumstances of each trip?
I share your view that most people would agree that a carry-on should be sufficient for a weekend trip, however I've still seen some pack horses with pretty heavy loads even for long weekend trips. Usually those are women with lots of grooming products and extra outfits, but you also sometimes see men who can't seem to trim it down at all even for just a few days.
For longer trips, it is much more of a mixed bag. There are clearly pros and cons to both sides and so what we're talking about is always going to be a trade-off of one set of advantages and disadvantages for another and each person will weigh the importance of each of those differently.
You mentioned some of the disadvantages of packing with just a carry-on:
1) Not having to get to the airport quite as early.
2) Getting out of the airport more quickly at the other end. You seemed to minimize the importance of the time saved and maybe it doesn't add up to much, but for some guys the perceived advantages of packing all the "stuff you want" is not as great as it might seem to others.
3) Also, as you already mentioned, not having to worry about someone stealing your checked luggage or otherwise tampering with it.
4) Even when your luggage isn't checked just standing around in an airport of bus station with extra pieces of gear to watch over increases your risk of having ripped off.
5) But what about the other worries of having your luggage lost or winding up in the wrong city. Again, these may not be huge risks, but they are there.
6) Something that might inrease the risk of loss or misplacement is if you yourself miss your connection and have to rebook on another flight. This is more easily done without any other additional worry if you have all your luggage with you.
7) Have you ever found yourself on an over booked flight and found yourself somewhat more reluctant to volunteer to be bumped in exchange for some subsequent advantage because of concern of where your already checked luggage would end up? Would you volunteer for an extra night in SJ, if all your clothes and personal effects were already on the way home?

If all you have is carry-on luggage that you can handle yourself to don't have to tip any skycaps or porters and increasingly, with discount carriers like Spirit Air, won't have to pay checked luggage fees.
9) Have greater flexibility to take inexpensive local transportation rather than expensive cabs because of all the stuff you're carrying.
What about the advantages of packing heavy:
1) Bring enough and you won't have to worry about running out of clean clothes or having to resort to having things washed.
2) Packing extra gear to be prepared for all contingencies whether they occur or not - e.g. do you pack a foldable poncho or pocket umbrella or even just pickup a cheap umbrella on the street if it turns out to rain while you're there or do you pack a full raincoat and wet weather gear, Do you pack a heavy sweater or jacket for your morning visit to Irazu or Poas or do you just layer up with your other clothes for the couple of hours you'll be up there, etc. etc.
3) Maintaining the flexibility to choose whatever you might want to wear at a particular moment by bringing extra clothes you might not wind up wearing at all.
What I have found is that this doesn't have to be an either-or propostion. The key is learning how to pack and what to pack to maximize the usefulness of what you bring so you don't really have to bring the extra stuff. If I can easily get away with just a carry-on bag, I'll do it every time. But if I think I need to bring the extra stuff that puts me over the top to the point that I have to check luggage, I will. What you'll rarely if ever see me do is pack REALLY heavy with multiple large bags. I packed for a month long trip this past summer, but I didn't wind up having to pack much more than I would have for a trip of just 1 week. Sure I could have packed clean clothes for every single day, but that would have seemed ridiculous to me when it is so cheap to have my clothes washed for me in places like CR. Here are my tips on how to pack so you don't even really need to pack all that much.
1) Start off with the smallest luggage that you can. If you start off with a non-carry-on bag you'll always manage to fill it up with stuff that you don't really need. Start with a smaller bag and you'll have to think about whether you really need all that stuff. If you absolutely do you can always switch to a larger bag.
2) Airlines do not usually count daypacks or briefcases towards your carry-on baggage. Carry large ones and use them to pack extra items.
3) Wear you heaviest clothes and bulkiest shoes on the plane, not in your bags.
4) Layer on clothing. A T-shirt under a shirt under a sweatshirt under a jacket or windbreaker gives you a lot of clothing options without having to pack them all. Packing many lighter pieces of clothing that can be layered is much more space efficient than packing items like a big bulky sweatshirt or jacket. This may not seem as relevant for warm CR, but I actually had to pack gloves, wool cap and long underwear a few trips back when I climbed Mt. Chirripó.
5) Plan clothing so you can wear anything you bring with anything else (2 carefully chosen shirts and pants = 4 days where you're not wearing the same thing). The way to do this is to wear neutral colors or pick one color family.
6) Consider how long it takes things to dry. Jeans may be great at home but if you get caught out in an SJ rainstorm, they may be shot for the few days it takes them to dry. They also tag you more as a gringo. Light slacks will dry out more quickly and make you pass as a local more easily. I don't know about you but a dark pair of pants will last me several days, but I go through shirts in SJ heat and humidity like there is no tomorrow. Consider getting a few quick-dry shirts or pants. They're very light and comfortable so you don't sweat as much and dry out quickly if you get caught in the rain. If you run out of clean things to wear, they can be rinsed out with a little soap and water in your hotel sink and hung up before you go to bed and will be completely dry the next morning.
7) If you're going for a week or more, figure on just having a few things washed. Hotel laundry services can be pricey per article of clothing, but you can sometimes just tip the maid a little bit to take care of it for you under the table or drop you stuff off at a place like the Sixaola and have it washed, dried and folded for you for under $4.

Bring dual purpose gear. Do you need a seperate gear for rainy weather and cool nights or will a water resistant windbreaker with a shirt and t-shirt on under it serve just as well?
9) Shoes take up a lot of space in luggage so try to limit them. Again wear the bulkiest ones on your flight. Athletic shoes are ubiquitous and are good for walking but if you want something that will be dressier consider getting something that are equally comfortable that will look as good with your dress slacks as they will with your jeans.
10) Rolling clothes rather than folding will not only help prevent wrinkles, but allow you to pack more into a smaller space.
11) Get small sample sized products, like shampoo, conditioner, medications like aspirin, shaving cream etc. rather than carry the full sized bottles or cans.
12) If you want to do some shopping at your destination, pack a collapsible bag in your carry-on that you can check as luggage on the return leg of your journey. That way you have the option of shopping and buying extra souvenirs at your destination, without carrying around extra luggage. Alternatively, consider packing old clothes that you can leave behind for CR's poor, thereby freeing up space in your bag for purchases made while on your trip and doing a good deed at the same time.
13) Electronics and various gizmos is the other area where it is easy for us guys to bulk up.
a) Do you really need to bring that laptop? Perhaps, for work or play. But how much time if any do you really spend playing those video games or watching those DVD's. Couldn't you just as easily entertain yourself during your flight with a book or i-pod? Do you need to bring that laptop for work or can you just bring the files you need on a CD and either burn the results on to a CD-RW or email them home? With cheap internet cafes on every corner, do you really need to bring an expensive laptop which can damged or stolen with you just so you can do your e-mail?
b) What do you really need for camera equipment? If we're really honest with ourselves, most of us just take snapshots. For that all you really need is fairly modest digital camera which are getting smaller and smaller and cheaper and cheaper everyday. Again, even ignoring the heft and bulk of packing the bigger fancier cameras, going around with big expensive items like cameras and laptops open you up to having them ripped off. BTW, I just got a new $100 slimline cellphone that can be used in CR, has a 2mp digital camera and music player that with a $25 memory card can play upwards of 700 songs and I don't even know how many pictures. That one item might be all the electronics I'll ever need to bring.
c) Other electronics. Do you really need to bring a ghetto blaster to set some sort of casanova seduction mood for your GFE or can you make do with the MTV channel on your TV? Do you really need that GPS to find your way around SJ or is it really more something that can get ripped off? Doesn't simply carrying the new easily packable ArchieLeach map make more sense than bringing fancy expensive and really unneccesary toys?
To answer your original question, for trips of a week or more it is naturally more challenging but still entirely possible to get away with just a large carry-on bag (or rather 2 - one for the overhead bin and an additional daypack) AND you can do so with out really giving up that much of any real importance. It is NOT just a matter of trying to save time at the airport or even about having things stolen while they're checked. There are a lot of other advantages to not being burdened with all that "stuff" while you're traveling around - less to lug around all over the place, less to worry about being ripped off from your person or hotel room, and greater flexibility in terms of moving around and being adaptable to changes in plans.