The issue with games developers is the same with movie directors. They are only as good as their last work and you can't bank on any future work being a success. The Call of Duty franchise, while profitable is really wearing thin. Treyarch did a great job with Black Ops but subsequent releases will be more of the same. Outside of that franchise, what does Activision really have? They are a publisher for other developers so not all franchises under their name will come along with the sale.
I'd say a better purchase would be either Epic or Square Enix. Their next-gen game engines shown at E3 are stunning. Apple could get a team like that to put together an iOS game engine that you get for free with the developer tools. Square Enix have such a talented group of artists but don't seem to know how to turn a profit.
Apple could build custom hardware components to accelerate parts of the game engine beyond what is capable in competing platforms:
Beyond this, they can make a real-time 3D engine for Motion like the Unreal tech demo shown:
Buying into any individual company in a competitive arena though comes with its difficulties. It's easy for the company to languish under the umbrella of safety of a profitable company and the purchase then becomes devalued. Companies shouldn't look to be owned by other companies but stand on their own right. Cultures can clash on merging companies together.