AMS prevents all damage taken from certain abilities. So it works like an immunity. E.g. with AMS up you run through the ray of 3rd boss in NH.
AMS is very powerful currently. And never forget the other perks.
There was a reason why mythic KJ kills had only Rogues, DKs and warriors, rarely a DH or ret. Because all of them offered important utility (also the top 3 had strong AOE burst).
Our pvp tree already has a nice talent option:
http://www.wowhead.com/spell=210918/ethereal-form
They could put in in t2 or t3 talent tree and enhance would have a niche strength (magic is always better, but at least physical works sometimes)