Well, I asked the same thing, although I was much more blunt about my dislike for it and got torn to bits by this forum. FFXI you had to /help if you wanted help and for the most part people respected you and your space, to include your monsters. I don't like it and I really don't like it when you are in a zone that there is no reason to do it. Starting zones yes, level 30 zones, no. Ill end there.
I guess it depends on the situation. However, me, I will never attack someone elses mob unless I know they will die or its an accident.
There are still quest mobs and hunting log targets in level 30 zones :)
I get it though, you're used to an idea that attacking someone else's mob is taboo, and habits are hard to break. But there's a reason why you can do it here, get full credit, and not take an exp penalty. It's a different game with different rules! There's no quest mob out there that's "challenging" or would live long enough for you to get some form of solo achievement. If you're fighting something much tougher than you, or higher level, and someone else comes and kills it with you... you
are probably the one killing his quest mob without regard to what that person needs. You scoundrel!
Seriously, test rotations on a dummy, then just notice if in real situations they work, but on something that is going to die in 30 seconds and someone else might be on a quest for (with limited spawns no less) don't get annoyed that they are getting credit.