First, At $51 billion, Apple's not exactly an easy acquisition. It would be cheaper to buy Sony, actually ($47B) and get a movie studio, music label, consumer electronics business, and a PC maker all at once.
Second, It's not as if Microsoft "needs" Apple. Microsoft isn't going anywhere anytime soon. It's got hordes of money, and a huge customer base that is largely locked-in.
By the same token, Apple's pulling in billions from the iPod, and the Mac market is stable and growing slightly. Office is guaranteed for the next few years: Microsoft itself said so. And the Intel partnership gives them access to whatever technology they can't build themselves.