I like wearing a uniform. Martial arts uniforms are made for martial arts. personally, I make sure that my street clothes will allow me to do anything my uniform will let me do (I even made sure I could do a side kick in my PROM DRESS, LOL), but the uniform is more durable than my street clothes.
Frankly, most clothes made for working out look sloppy. you have people in untucked t-shirts, pants with holes maybe, maybe ladies in sports bras and shorts. Martial arts has a certain dignity to it, and for the most part, that ought to be maintained. if someone is thinking about taking martial arts, they walk by a studio and see students dressed uniformly, lined up by rank and all that, as opposed to people in various states of dress, some with shirts not tucked in, it might influence which school they sign up at. The school that requires the uniforms simply looks more professional.
I have taught self defense classes where uniforms were not required. It was something that just kind of started all on its own... my best friend, who's gay, asked me to teach him some techniques, just basic stuff that could get him out of a situation should he need it. LA's a pretty liberal place, but after the Matthew Shepherd thing, I think a lot of people got worried... so anyway, he asks me to teach him. I tell him "ok. put on some sweat pants and a tee shirt, and meet me in the rec room." So we spend a few hours a week working on self defense... he tells his roommates what he's up to...and they want to learn. so I say "sure, bring em along" so I now have a class of four. Two of which bring their girlfriends to my next class. so now up to six. One of the girlfriends is in a sorority and brings some sisters to the next class... now up to around twelve. people start walking by and asking to join in... I had a class of twenty five college students... and me, being the businesswoman that I am and realizing that with this many people, I needed to have regular times set aside and couldn't base workouts around "whenever I feel like it" anymore, I started charging them five bucks a class. Great income for a starving student. They all knew I wasn't a black belt and couldn't promote them officially, or anything, and that wasn't an issue because I was just teaching orange and purple belt stuff anyway. The college didn't care what we were doing because we weren't using weapons, the rec rooms are first come first serve, and we were covered by the school's insurance. It was a very casual thing. Had I known what it was going to turn into, I probably wouldn't have started it. LOL.
I usually wore my uniform pants and a tank top, and everyone else was in various combinations of uniforms if they had em, sweats, board shorts, tanks, teeshirts etc. I'd wear a full uniform if I was teaching kata or wanted to do something a little bit formal and wanted to look kind of intimidating, but it was mostly just a fun little class to make walking around campus just a little bit safer. Had I been running a real studio where I was really concerned with making money, uniforms would be required. Uniforms bring a certain formality to a class that isn't always there without them. When I put on my uniform it changes my mindset from work, play, or whatever to "ok, its karate time now." and that mental shift makes a huge difference.