I find myself nodding in assent at the last comment... I also believe that the amount of code out there which would benefit from having an extra 24 32-bit registers is far more than that which would benefit from having 16 64/32-bit ones instead.
It seems more a matter of wording than anything in the CPU<p>" An assembler would translate the mnemonic “nop” into “xchg ax, ax” (opcode 0×90)"<p>But 0x90 also means nop, so it's not really translating anything. And now it still means nop in AMD64.