Also, not all people know or want to overclock, so for them it could be worth it.
I mean AMD Overclock and release something like HD6890, similar to HD4890.
WHat I mean is they could have done something better, possibly more shaders or higher clocks but the performance improvement is not good. However, we musn't forget the pricing. HD5870 sold fror 400$ +, here HD6970 is 370$ so in that respect, this card is actually earning less for AMD even though they had to spend on it's R&D.