I can't see any cons to having a swivel screen in a 5DIII, other than the perception that it would break easily. It would be very handy both for video, tripod use, low or high shots, or even discreet shooting.
I'm hoping for a 5DIII that will make my decision easy: a killer camera with 7D AF, same resolution as 5DII, better high ISO performance, usable contrast detect AF for video, rack focus for video, 1:1 pixel for 1080p video (crop) and/or pixel binning. Basically better everything in the same small package.
Although I believe it might be handy, certainly the reliability will be lower, and it might be more susceptible to moisture intrusion. There is always a trade off.