People are going to buy computers to do stuff. Email, facebook, ms word, that is what the average users says they do on the computer. One big problem with netbooks was that people assumed that could MS Word, and when they found out they couldn't they returned the computer. This is a solution where OEMs can expose people to other OS while still allowing MS Office to run. Maybe some people realize that they don't need MS Office. Maybe they don't want to pay a monthly fee for Office and realize that Google Docs or OpenOffice is sitting right there fore free. Maybe the next computer they buy doesn't have MS WIndows.
This is scary enough that MS, allegedly, has in the past prevented OEM from installing two OS. The last thing MS wants a computer user to know is there is another OS. Look at the misinformation on the Mac, how expensive it is, when my last Macbook Air was $1000. Yes, more expensive that they mythicla $300 MS laptop that runs everything, but about what a good laptop costs. We can argue price, but MS is scared of users knowing there is choice.
We also see this in past EULA in which certain versions of MS Windows could not be the guest OS. This is likely the future of the PC. A reasonably functional and free client OS on top of which a virtualized guest OS can be run. This is basically what MS is doing now with the instant upgrade. Start with a functionality locked out, and buy a full OS after the fact. Like the Mainframe manufacturers used to do. You have all the hardware, but have to pay extra to use it.