I have never considered the integrated grips as a plus. A *truly* integrated grip as those in the 1 series is actually a major gripe of mine.
This is one of those 'less filling/tastes great' debates with no right answer. For people who don't routinely use a grip, an integrated grip just adds weight and bulk they don't want/need. For people who do routinely use a battery grip, the integrated grip is a plus compared to the add-on (no flex so it's much better for tripod shots; better ergonomics, for example the second multicontroller on the 1D X is ideally placed, and it's location falls right at the junction of the 5DIII with it's grip, so it's in a different place on the BG-E11).
I happen to agree the grip can be quite useful for some. Even for those like me who loath it, some lenses are clearly better balanced on the 1 series.
Perhaps my take on it is based on shooting style... I personally have nothing over 400mm and that too just as a zoom that rarely sees use. I find the 70-200 2.8II heavy and too obvious and usually opt for the f4 which is only slightly less so... there was a time when I felt the 35L was too showy.
Even if I don't need to shoot at high speed or may not be partial to longer focal lengths, I may still want the perks the 1DX offers on the 5D3 (true AF coupled metering would be high on my list)...as things stand, my intern uses the 1DX while I favor the smaller body. There is really limited middle ground if one wants to move up the feature list without having to deal with the bulk and the grip.
Still, these are not major issues and I think we are spoilt for choices in the Canon camp.