Other than the unexplained endless poking from a few of my friends, the only reason I use it is to say "hey, I'm thinking of you, but I don't need a big ol' wall post to explain it." *shrug* It's not really all that useful otherwise, IMO.
In the past, I believe if you poked someone who wasn't your friend, they would be able to see a bit more details about you and some more pictures. Not sure if facebook privacy settings prevent this now. Anyway, it just sounds dirty.