You go public to have shares to trade for acquisitions. Most of the acquisitions made by Microsoft are actually very small, strategic purchases. Its only big ones have been Skype and aQuantive, and boy was the latter one an utter failure. You go public to get the attention of institutional investors and build brand equity. Does anyone NOT know what Microsoft is?
It would be difficult, of course, but it might be worth it. Microsoft has a complicated, multi-year strategy to execute. It needs time and patience, something people clearly do not have with Windows RT. What better way to execute than to do it outside of the impatient eyes of Wall Street analysts who only care about next quarter's projections. It's not easy to implement a multi-year strategy when four times a year you have to hear 'what are you going to do for me next quarter?'