My point is that the 5DII could be better than it is if it had larger photosites, and that's a trade-off that Canon has made to pander to those that seem to think that the solution to their photographic needs is to have more pixels.
Canon's release of the 5DII with the 21MP Full-frame sensor, was not "pandering" to anyone. The one and only reason why Canon released the 5DII with the 21MP was due to Sony twisting their arm by releasing the A900 Full-frame, with the 24.6MP sensor (announced almost a year back).
Simply introducing a 21MP sensor is just half the story. This 21MP will need high-end processors to crunch the massive amounts of image data that will come off the sensor - hence taking a hit on FPS. Also, once the data is processed, the data pipeline further downstream, will need to be robust enough to transport such gargantuan data volumes (especially when shot at say 5FPS at 21MP resolution etc) from the capture area to the buffer/CF-storage.
All of this high-end componentry that makes all of the above happen costs money, and Canon would have frankly preferred to have come out with a 16MP sensor than the more challenging 21MP sensor - believe it or not !
When Sony's Full-frame camera comes with a 100% viewfinder that is larger than the Viewfinder of the 1DSMKII and the Nikon D3, with IS built into the body thus stabilizing the 35mm primes, the 50mm primes, the 85mm primes, the 135mm primes, the 24-70 f/2.8, the 16-35 f/2.8 etc., along with 5FPS at 24.6MP resolution and pairable with ultra-high-end Carl Zeiss lenses (with Auto-focus), Canon too have to step up their game, since the "full-frame body marketshare" has a strategic impact to long-term viability of the system. When an electronics giant like Sony snaps up a traditional Photography company like Konica Minolta and then introduce a Full-frame a couple of years later, Canon knows that they don't intend to play second fiddle for too long and they have started to push hard......."pandering" was the last thing on Canon's mind, when they were forced to introduce the 21MP FF sensor in the 5DII, to be quite honest.