I have a lot of friends who have a negative knee-jerk reaction to the idea of paying a monthly subscription to a game . . . . either an MMO like World of Warcraft or something like D&DI. They've expressed similar sentiments as yours, they'd pay a one-time fee for digital access to a book, but not a monthly fee for digital access to ALL the books. (not saying that your own feelings are knee-jerk or invalid or anything)
So you'd pay $20 for access to one book? Why do that when that same $20 will get you 4 months of D&D access which gives you all books up to date? Even if you only purchase the "big three" each year (PHB, DMG, MM), you'd actually pay less by subscribing to D&DI than purchasing individual access to those books.
I think the current setup of $5/month (if you pay for an entire year at once) to get access to the crunch of ALL D&D releases plus the magazines is a steal! If WotC could work out a deal to include the PDFs of the monthly book releases (for a higher yet reasonable price, of course) I'd be completely in their thrall. I just don't get some folks aversion to the idea of a subscription for web access, especially when many already paid just about as much for subscriptions to the physcial mags.
Practitioner's 
