No one is stopping you from using other specs. I spec armor as a warlock. Also, someone landing a head shot with a sniper, or swapping weapons fast enough to blast you in the face with a shot gun means you are popping your super too early, or they are getting extremely lucky. Talking about melees, you should be jumping away and shooting well before someone is able to land 2 melees on you, unless its arcblade then your whole team is dead before you know what happened.