From my experience as a white-bread gringo who has struggled for 39 years to become as fluent as I am in Spanish, here are my opinions/observations.
Speaking Spanish is not as big an asset as one might think, nor is speaking no Spanish as big a liability as one might think.
Most hotels have English-speaking employees. The level of English will run from rough (Amistad, Don Carlos) to near-perfect (Presidente, SL). Taxi drivers will rarely speak English. Hookers: 1 in about 18 in the del Rey speak enough English to get the job done, including the nasty words and bedroom directions.
What you gain with speaking Spanish is a degree of extra safety and convenience, and better warm-ups with the girls.
Since this about men and women, sex and fun, speaking Spanish will often get a better performance out of your companion. Speaking Spanish may or may not get you better deals. I do believe that communication is key to great sex.
Here's an example. This last trip I was with a lovely tica who speaks pretty fair English. Things got hotter/better during our six nights together when I finally figured out that pillow talk, sighs and whispers in Spanish got her hotter in bed. This was initially uncomfortable for me and required mid-stroke concentration to accomplish, but I can tell you it was worth it!
What you lose by speaking Spanish:
1. If you speak good enough Spanish to carry on a conversation, you'll be much more likely to be treated to chica drama and bull*hit. Sob stories will quickly start coming at you.
2. If the locals know you speak Spanish, they will indeed often conduct their conversations very rapidly (beyond your ability to comprehend) or lapse into pachuco (common slang) to exclude you from the conversation. This is the only reason I can see that one might want to hide one's Spanish abilities.
3. Many native speakers will presume your Spanish is better than it is, particularly if like me, you speak better than you comprehend. This can be frustrating for both sides of the conversation until the native speaker figures out your actual level of Spanish comprehension. When they do... see point immediately above.
What to do:
If you speak no Spanish, here's what I would do:
1. Carry business cards with directions to your hotel, favorite clubs etc. and show them to the taxi drivers. Better yet, travel with a Spanish-speaking CRT wingman. Many of us speak at least passable Spanish, and there are several native Spanish-speakers in our ranks.
2. Find a trusted bilingual wingman to help you with negotiations. This works well particularly in a casual situation, like sitting around a table or at the bar at the SL.
3. Smile a lot and use gestures, hopefully not obscene ones. Nods and shakes of the head mean the same thing there as here.
4. Get a Franklin pocket electronic English-Spanish translator. They can be bought almost everywhere and for very little money. IMHO they're more convenient than a phrase book, which might also be a good idea.
5. Don't be afraid to ask your Spanish-speaking CRT bros for help. Most of us are helpful, friendly, courteous and Kind...sort of like perverted Boy Scouts.
6. Learn some basic Spanish, especially the numbers. As Don Gordo says, he knows enough Spanish to deal with comida (food), bebida (drink), mujeres (women), dinero (money) y taxistas (taxi drivers). To help you with this, try a language course on CD like Pimsleur. You can get the introductory 25 lesson package for under $20.00 retail. It's worth it if you've got a couple months before your trip and are willing to commit 20 minutes per day to learning the basics.
7. While you're learning a little Spanish, learn a little about Costa Rican social culture. For instance, nearly every friendly woman will call you "mi amor'" (my love). This doesn't mean she's into you... it's just their way.
Hope this helps.
Muy buena suerte / Very good luck.
Y bienvenido a CRT / And welcome to CRT!