I am TOTALLY with "okayfine" on this one. I think its pretty ridiculous. You are an audience member, not a performer. Your job is to experience what's happening ON stage, not to have anyone be thinking about you.
I do think it's funny that when someone wears a show shirt, they get grief no matter what. If its FROM the show they are seeing, it seems tacky -- like: look at me, I've already BEEN here. And if they wear a tee from a different show, others think its rude. "How dare they flaunt someone else's shirt here!? It's a no win.
Of course, t-shirts of any sort being worn at the theater are pretty tacky in general.
If we're not having fun, then why are we doing it?
These are DISCUSSION boards, not mutual admiration boards. Discussion only occurs when we are willing to hear what others are thinking, regardless of whether it is alignment to our own thoughts.