It have looked to its other mobile partners like Microsoft was playing favorites when a special deal with Nokia was announced. But then Google acquired Motorola Mobility and made Microsoft look altruistic in comparison.

Microsoft has relied on, excelled at and profited from hardware-partner relationships in the PC space. The smartphone space is much different - Microsoft's market share is much lower, and there are fewer partners and more stiff competition. But still, buying Nokia would disturb important mobile partner relationships with HTC, Samsung and LG, and Microsoft just isn't in a position to do that, writes veteran Microsoft watcher and ZDNet blogger Ed Bott in a recent .

Furthermore, Microsoft gets a heck of a lot of value from its existing partnership with Nokia. Despite its falling stock price and minuscule market share in the U.S., Nokia still has huge worldwide reach, which will allow Windows Phone 7 to scale quickly worldwide.

"Make no mistake about it: Nokia is huge," writes Bott. "Its share of the worldwide smartphone market is larger than any competitor -- five times larger than Motorola, for which Google is about to pay $12.5 billion."

And then there are the patents. Part of Microsoft's deal with Nokia is a cross-licensing agreement. Microsoft has access to Nokia's patents (for which Microsoft spent roughly a billion dollars), and Nokia in turn will be licensing Microsoft's patents. One of the main reasons Google bought Motorola was for its patents.