A few words again of someone who can accept the haunting system in special cases. First of all, for me its not really a system to enforce teamplay, but one to balance instant kills against raw damage (from casters). A quick example (numbers might be wrong, but i think they help to get the idea):
Assuming trash mobs on epic elite have around 10k HP, an instant kill is the absolute king of anything that can take down such a mob. Lets say we got an acid sorc that uses acid rain, 4 ticks per 500 dmg. He still needs to cast it 5 times (because we want to get down a group of said mobs) to kill his enemies. Thats 20 ticks, 1 per 2 seconds so 40 seconds until everything is down and he spent around 150 SP (+ probably some healing). A wail of a geared arcane would take out this group (we are assuming not too high saves, because it was said they shouldnt get too high to allow other abilities to work) with 50 SP instantly (so probably way less healing required), his wail is on cooldown for 30 seconds, but no enemies left, you can move on to the next encounter, wail is ready till then. In this case the instant killer has a huge advantage over raw damage. So, in such a case a penalty for this instant kill is reasonable. It slows down the instant killer because he is way faster than anything else without directly nerfing the instant kill ability.
As shown above this is only reasonable for high HP. And even if my numbers are completely wrong you should get the point that there is a certain amount of HP where instant kills are just way better than any DPS can be.
BUT (important part here) if enemies are below that amount of HP there is no reason for a massive penalty because other methods are not slower in taking down stuff. So haunting should take into account
As already mentioned before casters vs melees abilities in terms of taking down TRASH cant be fully balanced. Thats what casters are for, thinning out enemy lines or take down important enemies fast and leave the rest to melees. So if instant kills are "balanced" then against other magical powers.
- How many HP the target has (like -1DC 5 sec for 8k+ HP, -1DC 10 sec 12k+ HP ...)
- Which spell was used (Single target DPS might be quite a bit higher, so a FoD shouldn't lead to the same penalty like a mob killed by wail. Implosion can only hit 5(?) enemies and has one minute cooldown, again the penalty should be lower than a real kill all (wail) spell)
TLDR: Read it, lazy slacker!