Office is THE cash cow, OneDrive is (probably) a loss leader. Anything that can be done to ensure Office's continued near ubiquity, reduce usage friction, and shore up beach heads against competitors looking for an area to drive a wedge in will be done.
Just a short while ago, anything that could be done to make their products more integrated, and increase lock-in would be done. It didn't really matter if the benefited product made money or not. (For a very visible blow, look at the Windows Phone and Windows 8.)
OneDrive on Windows 8+ requires you to use an MS Account for your entire Windows session. So I'd have to pick a short password and do smart card support to use it. That's some pretty lame and obvious forced integration.
