One thing that really helped Halo 3 IMO was that when a new Map Pack was released, the previous one would go up as a free, required download. This let the people that wanted to support Bungie and Halo do so by buying early and having the new maps early, but it also greatly increased the number of maps available over the course of its life. With Reach, either Bungie or more likely Microsoft became more greedy, and none of the map packs have gone free.
To clarify, in Halo 3, its likely the Defiant and Noble Map Packs would be free right now since we have the Anniversary pack.
Personally, I believe I bought every Halo 3 map pack as soon as they were released. It as OK being able to play on them right away, but it was awesome when they would go free and everyone in Slayer had them, and I didn’t have to just see them in the DLC playlist.