I'll come out and say that i am prob one of those of "most people using that quickchat just change targets every gcd thinking they're hot s**t". But i swear to god that even if i have still not perfected my "target selection", it still makes more sense than seeing each of one of my teammates fight a different target, and for f. sake it happens one every 2 matches.
Usually, now that i am in diamond i wait a few secs to see if somebody else is going to lead (using quick chat, marking targets, etc.), but as long as nobody does i prefer to at least try to coordinate, and (as suggested in other posts and on PVPpaissa) if i see someone with 10% hp, 0 mp run away trying to use a potion or something i'll spam to kill him till my finger bleeds (while of course, seeing that i am trying to stop him myself i get cced by everyone and theyr grandmas).