Hi, the adult class I go to has a very flexible dress code, (basically what you find comfortable). The majority of the girls wear a black leotard and pink tights, one or two wear black footless tights or calf length leggings. The men on the other hand are a different matter. I am the only one who comes to class in black tights, sometimes i wear footless ones, and a dancebelt. The rest just wear sweat pants or shorts. Though my teacher makes them roll the legs up so she can see their legs an knees better. Have got to say at first it felt odd being the only man wearing them (guess you would feel the same if you were one of the few people inn class in correct attire).
But now I wouldn't wear anything else to class.
1 I think it looks more professional and dedicated. (I know most adults my self included do it as recreation, but I think seeing people in shorts, sweat pants etc, gives me the impression that they can't really be bothered, though I know that's probably not the case)
2. As the other posters have said, correct attire will make it easier for your teacher to correct your mistakes
3. I've found wearing the tights I am much more aware of my body position. When checking position in the mirror etc
4. Personally I find wearing correct attire, I am able to move easier, shorts and or sweat pants I feel hinder my movements, just because of the way they are made, with linings and the material.
5. This is just me, but one of the other guys in my class stands neare me during barre work sometimes and all I can hear when he is doing the work is the crinkle and rustle of his track suit bottoms. Which for some reason I can't stand

then I've always been odd
The thing to remember is, if you want to wear tights and leotard to class do it, don't let what others may think put you off. Just bite the bullet and goth class in them, it may feel odd at first but after the class starts, you will forget your wearing them, and will be concentrating on the teachers instructions.
Simon