Depends on where you are. I don't wear geek shirts to work (even though the dress code here is lax enough to get away with it), but I do wear them to conventions, for example. Sometimes, discovering fellow in-group members is exactly what you're trying to do. Tees of this sort are pretty good for that.

For introverts it has the added bonus of signaling passively, so you don't have to work up the nerve to start conversations with unknown factors as part of the search process.