That's the point in making it so cheap. Many more people will upgrade even if they don't really need to (also, a lot of Windows XP people who refused to upgrade to Vista and have been undecided about Windows 7).
I don't see much point in upgrading either
The goal is to have as many people as possible have Metro installed. Not only does it make supporting Windows XP less important, this takes the wind out of the competitors' sails on the tablet market. People usually want to have "the same" everywhere. Which consequently means fewer people will buy iPads, unless those come with Windows8, too. It also brings $$$ via the marketplace, and it secretly increases the install base of "secure boot" systems for free.
One day in the future, MS will then just flip the secure boot switch, and suddenly your computers are their computers. Class lawsuit following or not (which takes years), you'll not be able to run any other operating system (or software, thanks to Metro) in the mean time, unless they let you. Which means, if you pay them, directly or indirectly, and if they agree with the software that you want to run.
It's the same thing as with Visual Studio. The Metro-only version is released for free, as opposed to the "normal" one, because MS wants as many developers as possible to migrate to Metro, in as little time as possible. The easiest way of doing that -- much easier than marketing how awesome it is to have a screen full of small animated flashing rectangles -- is giving everyone a free animated-rectangle-creation tool and charging big $$$ for the desktop IDE.
And then... the entire worrrrrrld....! Muhahahahahaha!!!