Professional
Lets not forget that we're (actually, in my case "hoping to be") all professionals, and as such we deserve a small degree of respect from our students. Uniform is all part of the professional, clean-cut image which I feel all people, no matter what their employ nor walk of life, tend to wear appropriate, professional clothing - its the uniform?
The other aspect is that of perception. Two people, side by side, one wearing baggies, etc. and the other in shirt/tie, etc. You subconciously make a whole host of assumptions re: these people you've never met in a split second or two - and my bet is that I'd rather give my training bucks to the shirt/tie one?
Just my thoughts ...