I use Facebook so that I can connect with people I meet. If I don't exchange contact info with someone I meet, but they know my name, it's great for them to search and find me on Facebook. I don't want my Facebook connections to solely be people I choose to friend.
I guess it boils down to how people use the service.
Real Question: Why do you have an account, but not want people to be able to find you?
EDIT: Thanks for the responses, some of them make a lot of sense that I didn't think of initially.