From s development point this makes sense. From a marketing point this is a disaster. Customers want to buy, install and use. Having to download something again annoys them. Grabbing a new photopack would involve in-app-purchases. Again something putting people off. I prefer a single app.
You don't have to charge people for hte photopacks, no need for in-app purchases.
I doesn't annoy people for apps such as iBooks, ComiX, Kindle, Music, etc.. so why would it annoy people for your photobooks ? In fact, quite the opposite, what annoys me as a user is apps that are single purpose and clutter my springboard. Why would I want a "book" as an app ? I already have Kindle, Stanza, etc.. Why would I want another icon for a single book outside of these ?
Just asking you know...