Interesting as that has been my experience too...
I knew people in university who smoked from sun up to sun down and operated fairly well though they would have done better without the stuff.
I went to a well known uni where we had a number of now world famous athletes who smoked dope while there. It was before drug tests.
I always chuckle when I see them pontificating on TV when I remember what they did in the frat house next door...heh....
Mike Vick toked his entire redshirt freshman year away at Virginia Tech.