You really have to ask yourself, at this level, WHO is the target customer of each of these cameras.
The majority of professional photographers who need these tools in their arsenal (such as fps, extended ISO, full frame, high mp count) have very specific needs. The D3 was a great first step. But, in my opinion, it's targeted mainly at sport and event photographers. The D3x is geared more towards the type of photographer who is doing fine art, fashion, commercial work, where those large count mega-pixels are necessary for production. The D3s, again in just my opinion, is marketed towards wedding photographers or those who have significant low-light needs and HD video needs.
Each camera has functionality that is extremely specialized... and when you get down to the nitty-gritty, there is a reason why each one has the capability. And it's not for just for everyone.
As a sports/wedding/event photographer, the large files with 24MP would be overkill and it would slow down my workflow tremendously. But you better believe if I'm doing a billboard or a fashion shoot, I'd rather have the D3x on my side because the resolution is perfect for those types of projects.
I hope that all makes sense... I'm not doing a very good job of expressing myself in words this evening!
