Pairing people of similar level across a large number of campaign missions is probably very problematic. Public spaces and the social spaces are all wide-open matchmaking (one location, mingled player levels) while Crucible and Strikes are lots of people concentrated into a small number of playlists.
So the story missions are 1) content designed for one player, 2) will be gone through quickly and not revisited much as players progress (making for a shallow player pool) and 3 have players scattered across a ton of different missions.
You'll have long matchmaking times, and get players well above or below your level with crappy connections.
I'd like to see optional matchmaking for something like a weekly hopper of Heroic story missions, but it makes sense they are not there for the base campaign.