How so? Show me someone's blog subscriptions, and then metrics on entries they clicked on, how long they viewed it, and the clicks that they followed, and I could quickly get a very accurate profile of the person.
Assuming people were to ONLY use this for todo lists, that would be infinitely more useful than what feeds and stories a person is interested in. They probably also get a lot of that information already with Google News and Google Analytics.
I suspect there will be a lot more than just todo lists, so the possibilities are really endless.
Reader.. not so much.