As ATI’s primary goal to make Nvidia run for its money it plans to give partners more opportunities to overclock. Sapphire, HIS and many others will go stronger on overclocked cards in the next year.
The answer why is quite simple: the overclock card stands out in the bushel of reference ones. It makes a buyer want the card and this is exactly what both AMD and the partners want.
Nvidia has been doing this for years now, and ATI got there with some HD 2000 series cards and now even more with the new 3000 series. The goal is simply to increase the market presence and take away as many sales as possible from Nvidia.