I always wondered why they modeled the weapons left-handed and then never gave the option to use left-handed weapons. It'd be cool, but then I guess you'd have to flip the animations for player models aswell.
I read that most games developers model left handed weapons for right handed usage just so you can see the mechanisms of the gun and see how it ejects spent cartridges. Makes sense from a aesthetics point, but not from a realism one.
Edited:
Well the original counter strike (the mod) creators decided to make them left handed, no? I guess Valve didn't want to break the line.. or something. By the way the effect looks cool, though I miss that cheap point-type physics collision of the bullets. So cheap, so epic.
And if I remember rightly the CS mod team were mostly left handed or something, which is why their models are lefty.