Well, I still would argue that picking a camera first and then compiling a system around that is backwards -- start with the lenses instead. Cameras come and go, while quality lenses are a considerably longer term investment and should represent the larger percentage of a system's cost.
I wrote about considering the lens offerings of a manufacturer as being part of choosing a camera/ a system in my first post in this thread.
That said, I think you underplay the importance of the choice of camera.
You need to be comfortable with the way you work with a camera.
You need to like the characteristics of the files it produces.
So, yes, focusing at the camera is a good way to go, as all the big manufacturers offer a good array of lenses.
And both Canon and Nikon have enough to offer to satisfy a professional.
The camera needs to handle well and needs to allow to "sync" with it, so it disappears at a thing, and just functions like an extension of your senses and your imagination.
But your argument is valid for brands like Pentax or Sony. The latter is just building the system and you might not find everything you need.
Pentax is generally too limited in lens offerings, having seriously flawed lenses in their "pro" department.
