... as long as you buy them at our forum sponsors, www.MartialArtsMart.com

Seriously, it doesn't matter what you wear unless you are in a very traditional art, in which case you must follow the tradition. I do think you should wear something special, not only for the practicality (i.e. heavy gi's for judo) but also for what it does to your state of mind. The trappings can help you enter into a more liminal state, sort of like putting on the batsuit. I remember when I used to fence and do kendo, there was much ritual to the mask and kendo men (helmet.) Fencing as a gentlemanly art had many underlying honor codes that were acknowledgee with the mask. And kendo was a do - a spiritual practice - so the men served a purpose beyond protecting your head. It signified crossing over to the spirit practice realm.
At the same time, there is logic to practicing in street clothes every once in a while. But this should be done with the intention of training street techniques.
Additionally, school affliation is important, especially in the Confucian sense, so whatever the school uniform is must be honored. The meaning of the uniform is just that - uniform - so that nothing stands out about the individual beyond their skill. No foppish trapping, no whistles and bells, no ego, just practice. So when I do see someone break uniform, those trapping reveal their ego, and therein lies weakness.

BTW, anyone who know me know that I break uniform flagrantly. Even here at the office I've got the longest hair of all the guys (and some of the girls.)