I like to buy t-shirts for bands I like as that is money direct to their pocket, no label in between (yeah clubs take a cut..). To me, wearing a band shirt to the concert is also the further back the tour shirt the more legit (showing you are a long time fan). For instance I thought of wearing my Pure Phase shirt to the Spiritualized show, but didn't.
As to bands, I respect it when the bands are broke and that is how they get a clean shirt (or a reusable sweatty shirt for the show). If you are big-big, then put a shirt of an obscure band on, e.g. David Grohl with the Scream shirt on in the Nirvana video.