Ultimately this will be seen as a poor move as bold as it may sound.
Activision is tumbling in value, losing nearly half of its stock price in the last year (which made the move more affordable to Microsoft in the first place).
Now, much of Activision's value comes from being multiplatform, and not being multiplatform will shift to a significant decrease in returns, even if many people jump on board for hardware.
The thing here is that now Sony will put a new emphasis on creating their own Call of Duty competitor even with a smaller purchase of a company like Bungie. If Sony's Call of Duty competitor can sell even half of what CoD does, the return on their investment will be far greater than Microsoft's.
Microsoft obviously has deeper pockets than Sony, but these moves just aren't smarter moves. Insomniac for example for 229 million dollars... Think about that. That is the equivalent of buying Insomniac 305 times over...