Looks like Campaign Mathmaking will be in pretty soon but Bungie have a damn good reason for not putting it in on Launch.
From the BWU.
"We made mention of it earlier in the week, but since it wraps nicely into a couple of other points we want to drive home I'm going to cover it again. Right now, Campaign Matchmaking is not available as part of our online experience. The main reason for its omission is simple: we don't want your very first experience in campaign to occur on the very last level, and like all our matchmaking playlists, Campaign Matchmaking will offer all players in the party a vote as to which mission they'll end up tackling together. Democracy is totally sweet, but in this case, the majority might overrule your No Spoilers Doctrine.
As soon as we feel like there's been a big enough cushion between launch and present day, we'll open it up for everybody (more on timing in a few scrolls).
Other omissions are also likely owed to some inexplicable internal phenomenon too esoteric to cover here, but rest assured, we can and will make nimble and frequent changes to the matchmaking experience. That said, the scope and breadth of the Halo: Reach online experience is intended to cater to a pretty wide spectrum of tastes and gaming styles. While you should expect us to mind the experience, you shouldn't expect each and every one of your own personal whims to be catered to explicitly."
