I went out yesterday on a mini photoshoot looking for images for the Bad Apple book. Not the cover, that's done, but for a gallery at the end.
My suspicion is that no one particularly cares now about added content but next year it will be becoming an expected part of every digital book. It will be like a movie dvd, where people expect behind the scenes interviews, special effects secrets, and previews of coming attractions.
Some books will be crammed to the margins with video interviews of the author, perhaps music, maybe even games. If tradpub can ever figure this out, they will be way ahead of authopreneurs in this arena because they have the money to invest. Yet again, though, the whole reason there's a mass defection in paper publishing, is that people like Stephen King will be given the glory treatment and the average midlist author will get nothing.
No one should be surprised if authors say "What's in this for me?" to their agents and publishers. "What are you giving me?"
It will be progressively more difficult to convince an author that having their book on store shelves for 6 weeks before it's pulped, is better than having it available digitally until the end of time. Or to add in the financial equation, why turn over your digital rights in perpetuity, reaping the smaller percentage of the royalty pie while the publisher does nothing ever after except to give the author the pleasure of their company. Briefly. Very briefly.