I think it made sense in the 90s-00s rapid innovation space. Microsoft needed a computer on every desk/laptop/car/whatever in order to push Windows and Office licenses. Dramatic advances in hardware drove high turnover and more OEM licensing rev/profit.

Now that folks aren't upgrading very often, the cost of the Windows license is starting to be a much bigger proportion of the total cost, basically creating a price floor of PCs. This sucks if you're an OEM - you have a baked-in cost you don't control (aka MS Tax). Microsoft must naturally drop the licensing price to OEMS in order for OEMs to eek out some profit.

Long run Windows OEM license fees tend to zero. It's already happening with Windows 8 upgrade pricing (Mac OS X has already started this trend) and I expect it to continue.

MSFT knows this which is the primary reason to making their Surface Tablet (and Office online, to get SaaS recurring rev). I don't believe for a minute that they are only doing this as a "reference" implementation.