Well, I'm speaking from ignorance (I have no experience in publishing), but, to me, it looks like they're viewing the core books as evergreen and using the themes to introduce product for a time and I'm guessing will allow them to go out of print (or shift them to digital offerings). The bloat of product that others have mentioned as being a turn off to prospective players would be alleviated. Instead of seeing the core books and a handful (or more) of others, my guess is that they want one or two more products available at a time sitting beside the core. Then, directly beside that you would see ancillary products that directly support the additional products (minis that fit the adventure you can buy for example).
My guess is that they're looking at their numbers and seeing that they only have three true evergreen SKUs, the core books. Everything past that is some level of diminishing returns (just an assumption, but I think Ryan Dancey mentioned back in the 3.0 days that the PHB was by far the bestseller of the line). It's possible that their strategy is to support the game through timed releases that come and go without distracting from the Core. That certainly fits with releasing free player content to go along with the next theme instead of a full rule book.
Now, I believe that your argument is that Pathfinder is successful following the former model of having a steady release of content and D&D should continue with a model that is apparently successful. Your argument isn't wrong. I think the issue is that Wizards are testing to see if another model might be more successful. Earlier in the thread, someone mentioned that in publishing, it's the second printings and beyond where you find real profit. I don't know that to be true, but if we assume it, we can guess that Wizards is trying to maximize the return from their historically most profitable SKUs.
Or to put it another way, Wizards is trying to focus on growing the game (through brand synergy and outreach programs) and selling more core books instead of short term profits that come from selling new content to the existing audience that may discourage new players. The strategy may be flawed. I don't think we have enough distance from its start to really evaluate yet.
The test will be to watch their product schedule over the next year or two. If they shift gears towards a more traditional model (like Pathfinder holds), you'll be proven correct. If they stay the course, I will take that as evidence that their brand awareness pushing back to core has proven to be a solid model as well.
