HarryT, even if one is willing to download files for paper books one already owns, that doesn't mean one doesn't also buy new ebooks. The nice thing about books is that authors keep writing new ones. I for one am more than happy to keep buying new books I haven't yet read.
I don't quite have the confidence that Mobi will be there forever to keep re-registering new devices as old ones may break, though. I'm pretty sure I'll be able to display and read HTML and RTF forever, and I'd really rather pay for my books in those formats, or a format I can convert to those formats.
Even my old software is mostly still usable under emulators on my current equipment -- except for programs that had funky "copy protection" schemes. In those cases, I would need to rummage around to find "cracked" versions in order to keep using what I've paid for. I don't much care for a situation that requires me to break the law in order to keep using something I still hold a legal right to use.
The balance point needs to be in a different place. The enforcement needs to be focused on people who actually are breaking the law (e.g. file uploaders), not people who have merely bought a product and want to use it normally.
As for paying again for content I already own in another form, I know we disagree on this, but there may be a compromise point there, too, e.g. some kind of discount for those who provide some evidence that they own a paper copy. I really can't afford to pay even current Baen prices for every book I already own just to be able to access them while my books are still in the basement. But I could gradually replace my books for say, half-Baen-price, if they were all available legally in an open format.