Why
Your allies barely do anything except in scripted events. They shoot their guns at the enemy but it doesn't really matter because the enemy AI doesn't care. If you pop out to do something they'll drop what they're doing to shoot at you instead. Maybe it's because the enemy just shoots at the nearest available target or something but it's loving annoying when you have allies all around you but the enemies will still be waiting for you to pop your head out of cover so they can shoot it off. Then again, it doesn't matter whether the enemy kills your allies or not, there's an infinite supply of them waiting to take the other's place until the end of the firefight where a couple of them have an aneurysm and die anyway. That is, unless they didn't randomly die in the middle of the fight anyway.
Yes I know it wouldn't be fun to have your allies do the level for you but christ when it's standard human soldiers vs standard human soldiers the fight should be more even without being a total stalemate.
I recorded something in Fraps to show you what I mean, I'll upload it overnight.
Poll