Let's say Louis Wu makes an awesome custom game. The next time we're playing Halo 4 Spartan Ops and I say, "Hey, check out this sweet map Louis Wu made" and we go to a custom game and you don't have that Forge canvas because you never bothered to download it.. well then we can't play that map.

That problem gets worse when you try to run a customs night with 16 players. There's a very good chance at least one person wouldn't have downloaded the map.

And it's even worse when Louis Wu's awesome custom game gets put in matchmaking, but the majority of players would never see it because they never bothered to download the Forge canvas it was built on.

If this Forge canvas was a voluntary download, then Forgers would download it. I would download it. People that don't Forge probably wouldn't. You, for example, wouldn't. Why would you? I would venture that the majority of Halo players don't Forge and wouldn't download it. It needs to be auto-downloaded.