Disable second hand games?
It's more than that -- as others have mentioned it's generating more profit from each customer who purchases the console by tracking their spending habits in a manner that would make Facebook envious, if not Google. Their argument will be that by exerting more complete control over the ecosystem in this manner the end user will be guaranteed to have a much better experience.
Even though most people would voluntarily connect online anyway, allowing Microsoft and developers to obtain this information, they've probably calculated that a significant portion of people don't have their console connected to the internet just because it's not terribly convenient (maybe it's in another room) and they don't care enough to do so. They expect that the console sales they lose will ultimately be from low attach-rate consumers who weren't very profitable to them anyway, and will be outweighed by the extra revenue they can take in by forcing everyone to connect.
Additionally, by stamping out the second-hand market, Microsoft may believe they can appeal to developers in a way that Sony and the PC market can't -- publishers will have much less competition, and thus much more pricing power, because there will be no perfect substitutes in the form of the second-hand market for that exact game.
Personally, I absolutely hate it and have fully switched to PC gaming anyway, but I can see the business logic in it. For core gamers the competition between DD dealers in the PC space will likely result in avg. game prices being dramatically lower vs. the 720 -- in the long-run it will probably be the cheaper platform.
There is one major aspect to this that hasn't been discussed, though, and that's competition from mobile devices. As tablets replace laptops, a large portion of the addressable console market may be sated by an iPad, for instance, as "core" games become increasingly commonplace on that platform. X-com, for instance, looks like it will be fantastic on iPad, and there are rumors that Apple is creating a first-party gaming controller for iOS devices -- which is believable because it's low-risk and smart.