People don't wear sports jerseys to support a team, they wear one to identify with the team. That's why all the team jerseys have a player's name on it. (I've often fantasized of going ballistic when I see someone wearing one: "OMG, it's Albert Pujols, right here in my 7-11! You're shorter and fatter than I imagined.")
People I know who support teams wear their football shirts either when they're having a kick about, or when they go to games, Im pretty sure some of it is to do with supporting a team. Also, those shirts look pretty comfortable, I could go out on a hot day with a sports shirt on a it would feel pretty good. As for shorter and fatter, most people I see in star trek uniforms are scrawny with glasses, not exactly Captain Kirk.
The question is one of degree: a jersey or a Star Trek T-shirt: OK. A full Starfleet uniform (or full sports uniform): over the top and not OK.
Well, the poll is specifically about a uniform vs a jersey.