Two opinions on this:
1. Almost everyone currently does this with their tablets and smartphones. Why should Microsoft be any different from Apple, HTC, Motorola, etc... I bet that Windows 8 for ARM tablet will *only* be available to manufacturers, not to end users, just like the Windows Phone OS.
2. Maybe a possible future: antitrust action from the government forces all tablet vendors to open their bootloaders for alternative OS installs. Perhaps this would lead to people pirating Windows Tablet images onto Android Tablet hardware (?) ... I can't think of any other negatives.