Wouldn't you want the more dextrous hand to be the one choosing which notes to play? My sister plays string instruments, and she has commented many times that I am lucky for being left handed because lefties have a better time with complex fingering.
I suppose that it's easier to start out right handed on guitar, though, when the right hand is more active than the left hand.
I'm a lefty and I played right handed guitar growing up. I never got very good at strumming and picking. Five years ago I switched to left-handed guitars and I think I'm much better than I ever was as a righty. Picking the strings well, to me, is the most difficult part of playing a guitar.
I've never really liked that reasoning. If that's the case, then right-handed people should be playing "left-handed" guitars, with their right hands doing the fretting.
Personally, fretting with my left hand just doesn't feel natural. When I first got interested in playing, I asked a salesman at a Guitar Center about left-handed instruments. He handed me a standard guitar, and showed me the fingering for a G chord. It was uncomfortable, but that's obviously expected for the first time I'd ever held a guitar. However, when I flipped it around and fretted with my right hand, it felt much more natural. So ever since, I've played left-handed.
I suppose that it's easier to start out right handed on guitar, though, when the right hand is more active than the left hand.