If we were to assume for a moment that the 5D3 will come with a 24-105/F4 II and that the IS gets an upgrade in that, why do you want IS in the 24-70/F2.8 II?
The implication there is that the IQ between those two lenses would be roughly equivalent in order to keep up with the demands of current and future sensor technology.
Excluding DoF masking, 1/50 50mm @F2.8/ISO100 is 1/25 50mm @F4/ISO100 or 1/50 F4/ISO200.
Thus, maybe, just maybe, the feedback from the pro's that regularly use the 24-70 is that the extra price & weight of IS wouldn't add commensurate value to pictures taken.