I'm not sure how people are surprised by this. If you already have the track on your hard drive and now you have to PAY to listen to it then Apple isn't really giving you anything you didn't already have (convenience, obviously). It's a nice service, but realize that they have already lost the sale.
I would argue that you had a source file that may have been of lower quality and AAPL is basically giving you high quality rips of all of your music (DRM free) and hosting it so that your other devices can access it as well at any time.
