I've seen them done before, the split cleans, and all they do is put excess stress on the lower back, seeing as how you usually lean back when you put your leg back, and the leverage of putting the leg back is almost a "cheat" method in my opinion.
I don't have any real problems with them but they certainly won't provide the same overload that traditional power cleans would. 400Stud is correct about the low back stress and split cleans but it doesn't necessarily have to be the case if the pull is done with an almost purely vertical component. Single leg cleans could potentially be used for some specific strength development (if you felt it was necessary and if you felt that specific strength is of value). They could also potentially be used to correct power imbalances between the legs.
In general, I tend to think that there's no really bad exercises, only bad uses of good exercises. The ones listed above could probably find some place in a good program but it certainly wouldn't be for the same reason you'd use traditional cleans and definitely wouldn't be a full-time replacement.