Sorry, but Halo's single player game is far less interesting than, say, either NOLF, or HL. OK, so Halo looked better, but the rinse-repeat level designs were ridiculously lame. co-op, for the most part, alleviates this issue, as you and your buddy end up trying to solve identically-set rooms more efficiently than before.
and beating the final level on Legendary is worth the effort, but I don't have the skillz to pull that off without my co-op buddy to save my ass in a nasty firefight.
basically, until this patch brings back co-op (and with their netcode being unable to manage 4 people on a lan, I doubt it'll manage 20 critters), the best part of Halo has been carved out of the game. I'll stick to playing this on my Xbox, thank you.