Ron Paul 2024
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But for bands with smaller followings, it's just considered kind of tacky or just a faux pas. Like, usually you wear a band t-shirt to say you're a fan of the band. If you're at the show already, it's assumed that you are a fan, so wearing the shirt is just kind of redundant.
In my experience, (mostly in the metal scene) it's considered tacky for a band member to wear a shirt with their band logo on it. It's perfectly fine for fans at the show to do so, and makes the musicians happy.