Okay, anyhoo....as the mother of a special needs child and the classic "fussy baby" (Dr. Sears' Fussy Baby Book is my personal bible) I just have to say, there is a time and a place for them, IMO, if needed. I don't use one, but I won't rule out ever using one, either. Lots of other moms I know with autistic children say they use them because their children are fearless, run away, won't hold hands, and to top it all off are nonverbal. Slings are not always an option for these kids.
My ds still loves to be carried all over the place, thank goodness, and I love my Ergo and my sling. The time will come, however, when he WON'T want to be carried. And he doesn't hold hands, he has sensory issues with his hands. A leash may very well be in my future. It may not. Until then, though, I'm in the camp that says that every situation and child is different, and there are some very valid reasons to use one.