It depends on the person actually, when I port to Uldum I fire people up via general chat with messages “Leave none alive, slaughter them zugzugs” and on objectives if Alliance near me is not attacking the Horde I attack and they generally follow when there is someone starting the fire.
I never get close to those rares unless I have a party 
Vale is a horde place and it is good to avoid Vale rares.
Call to arms is ridicilous and harmful for PvP, it encourages low quality PvP. I have a topic currently going on, suggesting disabling kill counts in party for CtA quests. World PvP is fun when it is random, it is not fun when there is a group hunting single players with 5 people. So that people will have more 2v1 and 3v1 opportunities in which they can kill at least 1 player out of 2-3 instead of getting slaughtered without any chance of survival by 5 people.
It is the only option for now, that is what I’m saying, if Blizzard didn’t screw up faction balance by releasing disgusting races for Alliance it could’ve been a success rather than functioning as a permanent band aid.