For those of you that want real world times for comparisons as how long it takes to load from hdd -> hdd dvd -> hdd or straight from the dvd here's an interesting link.
The only problem with that is Bungie isn't worried about single player, and that's not the argument. It's multiplayer, those test were done on single player maps (who cares?).
"If you just plan to play campaign or offline content, you may feel that the tradeoff is worth it in order to have a slightly quieter Xbox 360 and reduced impact to your disc drive. However, if you play Halo 3 online, please be aware that increased map loading times can have a negative impact to everyone’s experience. In some cases, it could take considerably longer to load multiplayer maps, meaning that everyone you are matched with is also stuck waiting for your maps to load. If people start to lose patience, you then have scenarios of players leaving parties and aborting matching because they don’t want to wait. This adds to the problem, creating a cycle where more and more people have a harder time getting into the games they want to play. To avoid this negative situation, we are advising that Halo 3 players do not install the game to their HDD."
Bungie is worried that every one will get impatient with the slightly increased load times (if any) and jump out of matches left n' right the avoid a longer wait (possibly of a few seconds if any).
Some people have argued that if you have less wait in loading it's because you've been playing and it has already cashed so you will never notice it unless you play 7 games a day before you play H3. The problem with that is most people don't, quite a few people play nothing but H3 (and some other games every once in a great while).