Alien Swarm: is the armor useless?

I've been playing as a medic; provided the armour does reduce damage by 20%, it's saved me (and my team) from death many times.

However, if you are playing as (say) a officier, you are going to have much more hitpoints to play with and armour becomes comparatively less useful. I'd still go with electric armour, though, because getting infested really is expensive (especially on the healgun).

It depends on your class, play style and ability to swiftly move from the wrong end of a shotgun to the wrong end of a flamethrower.

As you start to progress to harder difficulties, you will ending a dedicated healer. These healing resources get depleted pretty quickly and you can only have one heal gun (unless you make someone else go healer and take one and swap when it runs out)

As a result, taking less damage is very important. It is preferred that your players are better at avoiding taking the damage of course, but if its still too much the armor never hurts.