Horace Dediu pointed out Microsoft's price conundrum on (which, if you are interested in Apple from a business perspective, you really should listen to).

You can see now how Apple positioned iWork components at $20 a piece... and these are analogous to the Office suite. ... How are you going to price Office for the [tablet] device?

While  for why Microsoft should go Metro-only on ARM-based devices from a technology and marketing perspective, Dediu notes the real reason the company can't cut the cord with Windows on its tablet OS:

I argue that they put it together like this because of the economics. Because they have to be able to charge $40 [for Windows] and they're going to say to the OEMs "Look, this is a PC. It's just a different form factor."

That's $40 for an installed copy of Windows 8. What happens when it's time to upgrade?