1. I shoot fashion work. No WAY I'd risk a shoot to get 3% more detail. If it's that important I'll go medium format.
2. I will likely be shooting my next film on the D800. No AA filter and having that sensor sampling at 2MP... the moire will likely be ridiculous. It's already brutal on the 5D and that HAS a filter and a lot less pixels.
3. The "E" costs more for very little gain.
More power to those willing to take the risk, or for those for whom it won't matter.