I am a casual user and have been since I was 16 or so. I'm 30 now and have really cut back in recent years but still partake a few times a week.
The biggest negative I could state in removing myself from it and looking at it objectively is the laziness factor for younger people. There are many young people who are lazy and lack ambition to begin with, and that 18-22 range when you're supposed to be figuring out what you want to do can very easily go 'up in smoke'.
Aside from that if you're a well adjusted person, I see no problem with casual use if you aren't doing it in the face of others (which is really dumb anyway).
Several times last year I smoked some before attending a game. Great vibes and high excitement level. I'd smoke one before entering and have a heroin beer or two and am on the edge of my seat the whole time.
