The skulls are just so arbitrary and random in their effects, I don't really respect them. My ideal of Halo combat doesn't involve having to melee flying drones every third round. If Firefight was legitimate, it wouldn't need such gimmicks.

It really needs objectives and counter-objectives. The Covenant need to be trying to move a bomb in or your team is trying to keep a door shut that would give the Covenant a flanking advantage. Just killing covies for the sake of killing them, I find to be somewhat numbing.

Even adding objectives might not be enough. Halo is best when it allows you to be bold and aggressive and it's hard to feel that when you're just sitting and defending.