Well... at 18MP the photodiode size is 4.3 microns. Using the rule of thumb for airy disk diameter (2.44 * wavelength * f/#) at 650nm (red light) at f/2.8 you've already hit the diffraction limit. Taking bayer mask antialiasing into account (which is not a perfect assumption because of the asymmetry with having more green sites than red or blue) you're still looking at hitting the diffraction limit at f/5.6. Most lenses don't even have resolution that reaches the diffraction limit, and when they do, it's at f/8-f/11. So basically, all of those extra photo sites are a complete waste. All they're doing is making the file size bigger, slowing post production, and making the noise even worse. But go ahead and keep arguing for the 7D... whatever helps you sleep at night...