Of course. I think thats great. I often borrow from the library now. At other times I purchase books. The problem with borrowing from the library is that they don't have an unlimited selection. If I want book A, and its on loan, with a 10 person waiting list then I have to wait... Or I could log into the Sony store and rent it right now. So, sure - free when that makes sense, but available for rent when that makes sense.
I don't like the idea of buying digital books for many reasons: 1) I can't pass them on after reading them, 2) its easy to loose your entire library by misplacing one reader, 3) the price difference between a printed book and a digital book seems small (or even non-existent), etc.