1. MSFT has cash in the bank and the Yahoo acquisition didn't go thorough
2. They are even more desperate now to bolster anything & everything to do with search & monetization
3. The perceived value of search companies (Powerset, Cuill) is higher since there are very few search companies that can allegedly make a difference to begin with, and even fewer buyers
Powerset is valued at 42 million (see venture beat article), if they are bought for 3x-5x that amount by msft and they can retain their independence they should probably go for it. If Powerset needs the same kind of computing resources, msft would be a better bet in supplying that.
Msft did something similar with TellMe in terms of retaining their independence apparently (no idea how far that is true).