OTOH look at the D3x and D3s, now the former has wayyy more MP but guess which has the much greater low ISO DR? Also the D3x that has wayyy more MP. So it's not so simple. If the 5D3 doesn't have a new readout technology then it could end up having 18MP and LESS low ISO DR, by a few stops even, than the D800. Hopefully that won't be the case, although the fact that marketing isn't already bragging about 2 stops better real and 3 stops better usable DR in RAW for low ISO makes you wonder a little.
Anyway just pointing out that it is not as simple as to say we need it to be low MP so we can get say great low ISO DR. What technology they use matter a lot more than the MP count.
I agree its more complex than that which is why I believe as its wrong to dismiss these stats(based on those grounds anyway) or to hold views such as "downsized higher resolution will always equal lower resolution on noise performance".