In addition, I believe (but could be wrong) that a lot of the differences between different cards boil down to the mode selection and initialization. if that is moved out of X.org into the kernel the X.org drivers get significantly simplified.
That isn't the Linux way. All your devices get moved to the new API or they are dropped from the kernel. Can't suck and blow at the same time.
I would not at all be surprised to see that in a few years the *BSD systems (and x86 Solaris if it is still around) add a compatible KMS interface as well, or be in the situation where new video cards are released, the configuration gets added to Linux, and the cards work great with existing X.org drivers (using KMS), but on systems that do not use KMS they won't work for some significant amount of time until the X.org driver gets enhanced to know about that specific card.