Because zerk is for DD warriors, I don't think you understand Warrior is meant for more than tanking, they straddle the fence as they can DD and tank both, so zerk when you have a PLD and Rampage when you're tanking, what is so hard about that?
EDIT: If you get hit with zerk up it resets. You aren't supposed to tank with zerk lmao. He is saying he wants -enmity so he doesn't take a hit so he can keep doing damage un-molested.