If Ballmer ends up owning an NBA team, it will be good for MSFT if he gives up his CEO spot - bring someone in that has a different but more relevant views/leadership - let's go MSFT. PS - Ballmer, do us a favor and help bring in new leadership - you've done well, and thank you, but shareholders can use the boost from your transition to the NBA.
Assuming Ballmer is a significant financial contirbuitor to a group that purchases the Sacramento Kings, my guess is that he would not leave MS to participate in the day-to-day management of the franchise since he knows nothing about the business of pro-ball; on the other hand, it wouldn't be the first time that he has assumed leadership in a business that he has no talent to run.
As I have said before on this site, if Steve had not been Bill Gate's roommate at Harvard, he'd probably be in mid-level management in the toilet paper division of American Paper, constantly regaling colleagues and customers with the story of how he got an 800M on the SAT.
If he does leave, it will be Microsoft's gain and the Kings' loss, that's for sure, but I'm not holding my breath.