You're thinking is very linear on this. Think outside the box and consider the capabilities of intuitive AI.
Lol. Look, I'm just trying to figure out what is the reason why people create individual profiles. When I turn on the TV, why am I inputing my own personal code, rather than just pressing the power button? If they could do something that induces people to do that, to tell the thing who is watching, and they could demonstrate that the sample of people doing this is not systematically different from the sample of all TV watchers, then sure, that would work.
But that's pretty much what Nielsen already does with people meters. They have a box plugged into the TV that tells it what you're watching, and when you turn on the TV, you're supposed to enter who is watching. So what is the big thing that would be gained by doing this with TiVo?