When you go to a friends, do you look at the books on their shelves? I do.
Do you look through their hard drives for any files that might be interesting? I don't.
I could get my PDF of the PHB up on my computer.
If I decide to show it to them
And I wasn't using my computer for something else
My friend can pick up my copy of the PHB off the shelf.
Even if I don't think about showing it them
Even if I'm reading other books, or on my computer.
It's a big difference.
And if DDI makes the books irrelevant, then digital books (being available only when on a computer, which could therefore just use DDI) are doubly irrelevant.
Devices like the iPad solve that problem.
You do not get the art or the fluff. As I said before, tying it to DDI and charging $10 to $15 a book would be acceptable for most people I think.
If you can't look at the PDF or other digital offering on the computer, you certainly cannot flip through it with DDI.