Why shouldn't Microsoft be broken up?

In the few days since Steve Ballmer's resignation has been announced, people in the media and tech press have been talking up the idea of Microsoft splitting up into two or more companies, as a way to enhance shareholder value (which has been almost flat since 2000) and allow individual businesses to operate under more streamlined management structures.

Now, can somebody please explain to me why a company as large and convoluted as Microsoft with distinct businesses shouldn't split up? The company's current formula obviously hasn't done very well for them in terms of keeping up with innovation and staying competitive in the market.

Here's a piece of the NY Times article announcing Ballmer's departure:

Ben Slivka, a 14-year employee of Microsoft who left in 1999, said the company should split up into five independent companies he calls "Baby Bills" devoted to Windows client software, Office applications, servers, Xbox and the Web. "Give each of them (say) $5B for a rainy day, but not much more," Mr. Slivka wrote in a post on Facebook after the news of Mr. Ballmer’s retirement. "You want them to be hungry. Return most of the cash hoard to shareholders."

Now, maybe five companies is quite a bit of overkill, I believe two or three entities is the best bet. The key here: "You want them to be hungry." That has really been the problem with so many of Microsoft's businesses over the years. hasn't it? The huge influx of Windows and Office revenue slowed their entry into smartphones and tablets, and the phone OS division continues to move at a remarkably slow pace compared with the Android and even iOS teams.

The way I think it should go down:

Windows, Windows Phone - Microsoft

Xbox, web services, devices - New company that Microsoft owns a majority stake in but is public

Cloud, office apps, enterprise services - New company that Microsoft owns a majority stake in but is public