They would be interesting to fight if they turn on friendly fire. Waaay harder to deal with incoming aliens if you've got to try not turning your buddies into lead holders. Couldn't just spray and pray, you'd have to form up, watch where you were firing. Part of the objective should always be how many make it out alive, I reckon. Have multiple levels with tasks that need performing and the option for perma death either per level or over the whole run. The more marines you lose, the tougher it is to finish, limited healing ability, wounds affecting performance - head shot and you're dead, mauled or shot leg, you're slowed, will your pals stick with you, or run off and leave you as bait?|
It could be good - done right.