There is no reason for seeing the legs.
Except that if I look down in real life, I can see my legs.
It doesn't break immersion
Well that's your opinion, I guess. It doesn't really make any sense at all that you think being able to see your hands and NOT your legs isn't immersion-breaking, but okay.
and it has no part in the gameplay aspect.
Yeah, I can totally imagine playing Mirror's Edge without the ability to see my legs. It'll be just about as much fun as the jumping puzzles in Metroid Prime.