From the two races I've been in so far, it seem to be common for the organiser to chuck in a "free" t-shirt as part of what you get for your money. I now have two. I also have two Cancer Research t-shirts, as they also give them away when you promise to collect donations for them.
These 4 "special" shirts are laying at the bottom of my runing pile in the cupboard, together with the c25k graduation t-shirt. I want to preserve them. Keep them "for a special occasion", although I know perfectly well that the special occasion won't ever come.
So what about you? Do you - or would you - use race-specific shirts, or would you like me keep them "for a special day"?