Closets are excellent if the people know what to do - the front two melee while ducking , with the back two shooting and then meleeing if they see someone pulled by a smoker, which makes it almost impossible to do damage to people in the closets.
The only situation where I've found the closet strategy a bit risky is in the finale on an expert campaign, where all it takes is one boomer to fuck you up when the tank comes, but the minimal damage from the horde is more than worth it.