I actually saw people on my server complaining in shout chat about others not helping kill their quest mob with them, after the other person had finished theirs.

I understand why you feel like that about others killing your quest mobs for you, it's a little annoying. I've had people kill mine in the past while I was handling it just fine and then send me tells that I was rude for not thanking them. Thanking them for what? Pressing tab and one GCD before I killed it already myself and never asking them to do anything in the first place?