The issue is that they have to have your size. If they don't, if no one else has exchanged their shirt in the size that you want, there will be nothing for you to exchange for.
I was in the process of my weight loss when I signed up for TOT last year. I estimated what I might be, and I estimated too big. Add to that the fact that the TOT shirts ran very very very big last year, and it just wasn't good. Because they ran so big, I think a lot of people were exchanging for smaller shirts, so the smaller shirts bin was empty most times I passed by. Finally I saw a shirt in the right size bin and ran up. They recognized me from my other pass-bys and grabbed it for me. Yay.
If I had needed an exchange at WDW marathon weekend this year I would have been out of luck because I couldn't be there very long. Wanted to be, but was really sick and needed to get out of there asap. (sorry for the germs...so sick I wasn't thinking clearly) I estimated well this year. And thankfully the shirts for the Half weren't sized anywhere near like the TOT shirts were.
So it wasn't that it was a bad experience. They just have to have the shirt size you're exchanging for, and at TOT last year they didn't. For awhile.