I know I'm starting to sound like a broken record, but ...
1) Some software packages, like DxO, can't process sRAW & mRAW, so some photographers would be forced to use the full sized raw even though they don't want or need the full resolution.
2) The APS-C sensors today are pretty close in resolution to the 5DmkII - the 600D, 60D, and 7D are all 18MP. It seems Canon is increasing the resolution on the APS-C sensors as well. I don't think 5DmkII owners would be happy to replace it with a 1100D.