Razz, I'm sorry, but I totally disagree with you on this. There isn't a single book outside of core that sees as much airplay as a module.
Take a module that lasts for 50 hours of gameplay. Not a huge one, but not a small one either. You have to figure that every player at the table is using that book for the duration. The players are playing it and the DM is running it. For the stock 4 players+1 DM, that means the book is seeing some 250 game hours. Nothing you could buy will come even remotely close to this.
Sure, you run it once and then put it away. But, how many books to you pull one or two ideas out of then stick up on a shelf never to bother with again?
For me, the bestest thing is modules. Particularly the big ones where you get pretty much an entire campaign in one book. I look at the hundred bucks I spent for my World's Largest Dungeon as the best gaming money I've ever spent. 180 in games hours so far and probably another 90 to go. Multiply that by my five players and I couldn't possibly buy any other supplement that could compare.
Gimme ten modules like WLD, Son's of Gruumsh and Red Hand of Doom and I won't have to buy another gaming book for the next decade.
Can I say, "Please sir, can I have some more?"