I wouldn't presume to put words in Ballmer's mouth; I'm just a home user who's been playing around with MS operating systems since the days of Win3.0. (Now that was a POS.)
Curiously, Windows had something called "Windows for Pen Computing" way back in 1991. It ran on Win3.1. It went nowhere. It was also available for Win95, WinXP, and its capabilities were built into Vista and 7. But tablets didn't get popular until 2010.
Likewise, Windows came out with Windows CE, which was mainly used for PDAs, in 1996. There was a version aimed at smartphones that came out in 2003. But it never saw the "smartphone in every pocket," "gotta have it" kind of success that Apple saw in 2007, when it released the iPhone.
So it took Apple to make both small, all-in-one pocket computers, and tablets, successful, leaving Microsoft to play catch-up. (The state of miniaturization, the ability to put a system on a chip, also probably had something to do with it.)
As far as Windows 8(.1) goes, the Metro UI has some compelling aspects to it. Lovely, multi-page views of news articles with unobtrusive ads. Weather reports with all the "weather porn" you could ask for. Full-screen maps. Free, downloadable games, nicely done, with unobtrusive ads. Etc, etc.
The problem with it is that it's a tablet/smartphone UI that's bolted onto a traditional desktop UI, and transitioning between the two is awkward. The visual difference between the two is jarring. Seeing parts of the Metro UI come up on the Win8.1 desktop when you mouse over to the upper right-hand corner of your screen is jarring. IMHO, Microsoft should have come out a different OS for tablets and smartphones, and left its successful desktop UI alone.