It has not been specifically mentioned in this discussion that the large pixels of the 1D3 (and some other bodies) raises the Diffraction Limited Aperture to values much more forgiving in the field. The 1D3, 1D X, and 1Ds3 all come out to f/11 or slightly narrower; the 5D3 to just a touch wider than f/11.
The smaller pixels of some other bodies cause the onset of diffraction to occur way down the aperture scale .... 7D and 60D at f/6.9, 50D at f/7.5, 70D at f/6.6 and the Rebels at f/6.8 to f/7.5.
Having owned and used a 50D and 7D extensively I ran into this trying to stop down to f/11 and f/16 for landscape shots. Any advantage of more pixels was wiped out by this problem. The very small DLA made using some lenses difficult if they only achieved acceptable sharpness at reduced apertures.
If you read the entire set of articles referenced above you would notice that the large number of pixels in the bodies you mention (1D X, 1Ds3, 5D3) compensates for the individual pixels being somewhat smaller than the larger pixels in the 1D3.
The author of the articles speculates that somewhere around 5 to 5.5 microns is the sweet spot, higher if you especially favour low ISO and clean files, and lower if you want more resolution above all else.
Several bodies like the 7D (4.3), 50D (4.7), 60D (4.3), 70D (4.1) and the Rebels (4.3 to 4.7) fall on the "lower" side of this scale. Nobody complains about the resolution of the 7D; but we do read (and I experienced) the struggles to get "clean" files (it drove me crazy!)
We all know your 1D X, 5DM3 and 1Ds3 are better than a 1D3 ... why wouldn't they be? They are all professional level like the 1D3, and are either newer, more expensive, or both. Like you we would all use a 1D X instead of a 1D3 for action or a 1Ds3/5DM3 instead of a 1D3 for portraits and scenery.
But not all of us can own all four of these bodies ($13,000 to $14,000 I figure). Most of us in this forum have a 1D3 and one of the others at best ... that's my situation, and here in Canada, at my club, I am already considered "camera wealthy" by many ...
The point I am making is that for a great price used (about $1000-$1200) a shooter can get a versatile professional level body that can produce images close to the "big boys" worth much more (1D X, 5D3, 1Ds3, 1D4) and as good or maybe in some cases better than some of the others (7D, 60D, 70D, Rebels), and that part of that is due to the "giant" 1D3 pixels.
Thanks Max, this is a good detailed and can say brief details of what i may read on that link given to me.
Well, i never worry about that pixel density or size when i buy any camera, i only go with overall or whole specification and see if that will suit me, some people buy a camera for only 1-2 specifications, but for sure i understand why many go with less expensive body that has good enough performance such as 5Dc/5D2/6D/1D3....etc
I really feel lucky in Canon bodies, i only don't have 1D4 or 5D3 to have really amazing bodies collection even i have 1DX and 1Ds3, but doesn't matter if i can buy another body later that can be in a place of 1D4 and 5D3, 1DX is in between of those 2 bodies.
Believe me, i was happy using my 1D3 for sports before i can get 1DX, it replaced my 1D2n with no mercy regardless of all those AF servo issue, and i still keep it as a backup, but i was planning to sell it so i can get another 1DX or at least buying a used 1D4 to have that crop factor but newer model than 1D3.