This is actually the reason I pirate all of my kindle books. That, and at the rate me and my girlfriend read books, it would be completely not viable economically.. we're talking 3-4 books a day being read, That's much, much more money than any other general hobby (think, WoW is $15/mo.. a new GOOD book ebook/hardcopy is 3-13$)
My system is as follows; If the book is good enough that I'll want to read it again down the line, I'll pick up the paper or hardback copy for my bookshelf.. not only does this provide great asthetics but allows portability, and allows me to loan it to friends.
The kindle has more than paid for itself in a month (We have exhausted the library's supply a good while ago)... I'm sure there's plenty of people who consider what I'm doing 'wrong' and that's fine with me... I just like to read, and I can't afford to spend 100$+ a week on books.