They've already stated that the max they'd consider is two people. Otherwise it's like running multiple tanks through a china shop.
One of their focuses is on making the game fun through balanced gameplay. If you add another spartan, you'd need to roughly double the enemies in-order to maintain the gameplay difficulty level.
However, sometimes the challenge doesn't come in the fact there's multiple enemies, but how they're arranged and what kinda' cover you have availible while fighting them. In that scenario, it becomes significantly difficult to balance the gameplay.
The concern is that for some folks, the only way they'll get to try-out the game to see if it's worth buying, will be by playing it briefly at the video games counter. If the co-op is geared towards multiple players, then it'll suck for single players, and vice versa.
If someone joins the game in-session, do you immediately double the enemies? or do you immediately make them all smarter? or does it become an instant slaughter?
maybe if there was some kind of flood mp game (like invasion on ut2k4 for those who have played it) where players join at the end of waves of enemies. then the number of enemies will naturally increase in number and difficulty. I know they already said no for now, but maybe its a possibility in the future, similar to the map pack they put out for halo 2.
I have to say I'd really like the option of drop-in/drop-out Campaign co-op, but I'm not gonna hold my breath on that. And it would be problematic w/ balancing the transition in enemies and difficulty. It'd be a big enough of an improvement to have Online co-op.