Yeah, but we are talking about the 360. You can't just slap any hard drive in that thing like you can a PS3. You need to buy a proprietary MS one. And while they have gotten a lot cheaper in the past year or so they are still much more expensive than the average hard drive. Most of the time twice as much.
Of course you don't, and I've specified that a few times. But Carmack himself said that if you don't some of the textures will look shitty and may not load and there will be a lot of pop-in.
Actually, I would say the complete opposite. For one, multiplyer is just that one off race mode they talked about. I hope you don't think it's going to be death match or anything like that. Plus multiplayer games usually look a little worse than the single player anyway. One of the main draws of this game is how amazing it's going to look thanks to the new iD engine. Not installing the campaign discs would be doing yourself a huge disservice.