That said, I wish Scribd and their writers the best of luck.
Or get a better advertising company: [Shameless, but very muted plug] :-)
[Edit: this is strictly for the non-fiction trade publications, the sort of stuff that you typically see on Scribd. Fiction might have a better future in ebook readers and their associated online shops]
iTunes has managed to sell an incredible amount of content, even though it doesn't make much sense to me or you. I think in the past I've overestimated what an individual will pay for the ability to Buy This Copyrighted Content. I guess time will tell if this model can apply to text-based content.
It makes a lot of sense to me, actually, because you go to iTunes precisely because you want to pay, and they've built up a user base of people who are willing to pay for music. This user base is not coextensive with people interested in downloading music.
Scribd has built up an audience of users interested in reading things.
(More broadly iTunes is a footnote on the gigantic tome of marketing success that is the iPod, and their profits are rounding error next to the money made selling 4 GB hard disks to people who are probably not spending $20k to fill them, but that is neither here nor there. Even if Apple lost money on every track they'd make it up with volume through the plausible deniability gained.)
I consider the music, movie and now literature based problem to be basically the same issue regarding digital distribution. Any problem that was present with music will be a problem with the others. I wouldn't be surprised to see iTunes selling books in the future.
If Scribd can provide some pressure to make content less expensive or give people more options, I think that's as good as Amazon's mp3 store was towards making iTunes DRM free.