Well, I will make the following argument:
AMD is trying to sell CPUs, and they want to draw away people who will buy Intel. So what better way to "beat" Intel in this multi-core frenzy than to..."out-core" them! For marketing purposes, nobody (i.e. Joe Blow) cares about whether the 8 core AMD gets decimated by a 6 core Intel. All they care about is "having more cores".
In fact, I was spec'ing out a system for my buddy. He went with an Athlon II x4 (for budget reasons) and would constantly tell me that he got a "quad core" for much cheaper than I got mine. Then he bought an 1100T and began saying, "Wow, I got a 6 core for $200! Still cheaper than your i7!"
Despite my explanations that per-core and per-clock, my CPU was faster, hence the higher price, he didn't care 'cause he had 6 cores.
Most people are like this, and AMD is going after sales by marketing "cores" like this. And I believe it will help them because why should I buy Intel's $600 Gulftown when I can get AMD's FX 6110 for only $240?
Clock-speed race, anybody?
EDIT: And they would ABSOLUTELY sell their top end chip for $999 if it performed at that level. They did so in the past! Because if it really did, they could price their other chips accordingly (FX 8130 = $999, FX 8110 = $600, FX 6110 = $399, FX 4110 = $200) . But these prices (which I'm becoming more and more convinced are legitimate) preclude this possibility, unfortunately.