I must be missing something - why not put the IS in the camera instead of putting it in each of the lenses?
Sony, Olympus, and several others do just that. It has the advantage of working with any lens, in theory. In practice, in-body stabilization is much less effective with longer lenses (the sensor can only be shifted so far and so fast), doesn't stabilize the optical VF, and doesn't help with AF since the AF sensor isn't stabilized. I suppose Canon and Nikon would also consider not being able to charge more for IS in each lens as a disadvantage.
Speaking of missing something, despite Sony being big proponents of sensor-shift IS, that feature is missing from their new FF mirrorless a7 and a7R.
One reason I'm keeping my Olympus OM-D and hesitant to try a Sony 7/7r is the excellent performance of the OM-D's IBIS, which stabilizes the EVF (if you're lucky enough to figure out the correct setting...) and, because it's mirrorless, doesn't need an AF sensor; it works superbly on the 100-300 Panasonic lens (better, probably, than the IS in that lens, which I keep turned off). This is not only good for "native" m43 lenses, but wonderful for legacy lenses - it's easy to manually focus an IBIS-stabilized 135mm legacy prime on my OM-D, quite an ordeal on my Fuji x-e1, despite the reduced crop factor (there's no point even trying on my FF Canons).
I seem to be able to get focus on my legacy 135mm just fine on the a7, even with the viewfinder at 5x magnification. Getting the shots sharp is another matter shutter wise is a problem, but often I find that people move more than I get hand shake, so that higher ss is not a bad thing. Also, the sensor can be pushed much further than where i'd push any m4/3 to ISO wise.
It's all compromises, I guess