With Readability you still have to "remember to click" the read now/later button, and you don't get to choose how much you want to support that article; you actually have to support it _before_ you even read it.
If they don't have one already, I expect an opt-out feature before too long. It's the first objection raised to my system whenever I talk to people about it.
In terms of the business model, Contenture was almost exactly what I thought up on my own.