While it is certainly true that the "big" desktop distros are all based on initramfs/initrd "preboot" environments, this does not mean it is an equally well-suited solution for all Linux installations.
Obviously, in situations where the initramfs approach works, the traditional roles of /bin, /sbin and /lib as a "rescue system" are no longer required.
On the other hand, the initramfs approach requires a rather large amount of main memory in order to store the extracted contents of the preboot environment.
This is certainly not a problem for the current generation of hardware where most boxes have at least 1 gig of RAM.
But there are still a lot of smaller and more restricted older systems out there with as little as 16 megs of RAM!
Those systems can still run a (somewhat stripped-down) Linux installation (typically 2.4 or 2.5 based), but they certainly cannot afford to extract and run an initramfs.
Such systems *depend* on the traditional /bin /sbin /lib layout for a rescue system, especially in cases where alternative boot devices are not available.
Merging /usr/bin and /bin is therefore certainly a valid choice for bloated desktop distros, but I strongly advise against general adoption of this scheme in a forthcoming update of the FHS.
What *could* be adopted, though, is the *optional* of replacement /bin, /sbin and /lib with symlinks.