I think you're correct on the value of these kind of features for a box that is being subsidized in the living room. Obviously a number of companies (Apple with the AppleTV, for example, cable companies with the PVR services they offer, TV manufacturers with Smart TV junk, etc) are circumspectly trying to follow this model, and obviously if a company can actually form partnerships with cable companies to replace basic cable boxes or PVRs with their own stand-alone CE device, that'll be a big get.
The thing I'm skeptical about is Microsoft's ability to form those partnerships--or anyone, really. I think the service providers Microsoft would need to partner with to get this working are very, very, very scared of being muscled out of business by other content provision avenues; put simply, I think Comcast is scared to allow companies to control this element of their business.
You correctly compare this to the carrier-smartphone manufacturer relationship, but one thing you don't mention is that by far the most successful smartphone company basically came in and ate carriers lunch... and they're not really happy about that. I can't imagine another set of providers are going to be happy being reduced to dumb pipe and marginalized.
Even with my skepticism I think Microsoft will be able to achieve modest success here. With no partnerships, they'll still be able to act a sort of passive receiver, and you correctly note that Microsoft is able to self-subsidize the device (SaaS/HaaS model).