Speaking as someone who bought every single 3E Eberron book to come out, and someone who rarely buys 4E rulebooks (why? I have a D&D Insider account), but does buy the setting books...
I'd say I like the approach somewhat. I was kind of miffed by the Dark Sun player guide/campaign guide being put into one book. Honestly, I'd like to have seen a Campaign Guide, Player's Guide and Creature Catalog.
But, that's not the point.
My main point is, I'm fine with two or three books and out. But, I want more content, adventures, locales, NPCs, organizations, items, and other setting-specific stuff in D&D Insider.
They've done a pretty good job with Dark Sun. There's been a good deal of articles lately. But, I worry that that'll end. FR and Eberron haven't gotten a great deal of support that I suspected. Dark Sun may dwindle off after the "release honeymoon" is over.
The more settings released, the less resources they can devote to each in D&D Insider. It's a delicate balance.
So, in sum, I'm fine with 2 or 3 books for a setting (or, one larger book). But, give me more to fill in the gaps with D&DI.