Well, first of all, disposable diapers stink, even when they're not dirty. Cloth doesn't stink unless it's dirty.
Second, I don't wear chemical-filled paper underwear, so why should my baby wear chemical-filled paper diapers?
Third, they're cute. Really, really cute.
Fourth, when my baby poops, it's all contained in his cloth diaper and doesn't blow out the back. I used disposables on 2 of my kids before I discovered cloth, and let me tell you... there was always poop up their backs.
Fifth, they can be a real money saver. (Not for me because I'm a sucker for the cute prints and have way too many, but oh well.)
Sixth, you'll never catch me running to the store for diapers at midnight because we ran out.
I think that about covers it.