The way I see it there are advantages of increasing resolution, probably at least to 50 MP, but I also agree that the resolution advantage is easy to waste.
Here are the advantages of increasing resolution:
- Better detail at optimal aperture
- Less aliasing, so OLP filtering can be reduced or with the D800/E eliminated
- Less jaggies on thin lines
There are really only two "down sides" of increasing resolution:
- Increasing resolution will reduce DR somewhat, doubling resolution (from 25 to 50 MP) will reduce DR by 1/2 stop.
- File sizes increase
I don't think that the 36 MP the Nikon has compared to the 5DIII's 22MP does change the world, but it is certainly an advantage for the Nikon. Nikon probably also has a healthy lead in DR, due to their Sony Exmoor based sensor. Although little data has been seen for either system I'd say extrapolating from images and data regarding D7000 and D7 is a valid method.
In my view, the upgrade Canon made is a welcome improvement for Canon users, on the other hand it is first with the D800/D800E Nikon owners have access to a high resolution camera at reasonable price and superior DR at low ISO.
Sony Alpha 77 and NEX7 has put 25MP in APS-C, that correspond to 52 MP on full frame. Now, dream about a nice Alpa body with a Phase IQ248 with 148MP CMOS sensor from Sony. Wouldn't that be a nice dream? Except the price tag, of course, a real nightmare!
I'm another who is of the opinion that more than 20-something megapixels in a 35mm full frame format is pretty much a waste. That's simply because of the small sensor format and the huge amount of magnification required to take it to large print dimensions. Lens diffraction, focus inaccuracy, and every other quality issue are magnified equally, so it's between difficult and impossible to get much more than 20-something MP of true resolution out of any 35mm format sensor. You certainly can get more pixels, but they don't necessarily translate into more information or better print quality. I'd think those who truly need more resolution than a good 20+ MP 35mm format camera can provide would be far better off spending their money on medium format. It costs considerably more, but an increased MP count in a larger format makes a difference that can actually be seen. Perhaps 30 MP would be nice, but it's on the far outside fringes of what's usable with the best prime lenses (certainly not any zoom), stopped down to their optimal apertures (say F5.6 or possibly F8), mounted on a camera that's fused to bedrock, or at least a very heavy tripod. Not many people actually shoot like that. Of course all this is just my opinion, and it along with $5 will buy a fine cup of coffee somewhere.