I personally wear my letters a lot to this day being an alum. If my lavaliere isnt around my neck 24/7 then its on my wrist. I also have about 15 shirts, just ordered a windbreaker, and I am about to order a long sleeve-fitted shirt from greek101. That's from being a collegian just one year! At my school I wouldnt say that letters were/are worn a lot, but you see them pretty often. The only designed days for all Greeks to wear their shirts are Greek Week and Go Greek shirts for the first 2 weeks of school. Going though one year at school you can accumlate this many at UM...

Go Greek Shirts
Rush Shirts (Fall and Winter)
Bid Day (Fall and Winter)
Barn Dance Shirt
Semi Formal Shirt
Date Party shirt (2)
Formal
LXA Watermelon Burst Shirt
EX Derby Days Shirt
DG Anchor Splash Shirt
Greek Week Shirt
Greek Move-In
Work Week
Theme Party shirts (lets say 3)
Thats 19 shirts one year if you wanted to buy them all and 76 total for 4 years. Not too mention the random stuff you buy on your own and other occassions like Senior Bar Crawl and Tailgates/Pre-Parties. No wonder you see a lof ot shirts on ebay
Greek Week Team Shirt