I think ur miss-understanding what i mean by subscription.
Its the same model as been a member of a library, your allowed a limited number of books each month with the garanteed knowledge that you will give them that much money each month they can buy more books.
look at this http://my.safaribooksonline.com/ for an example, one that in my mind one that isn't there yet!
When you've got a 12month+ income stream that is certain, you can buy a lot more for a lot less. That to me is how these things would be brilliant, much the same as a public library that you can instantly access anything of, that you can top up if you want with the "premium" stuff its lacking for considerably less than the paper back.