There's a small issue with this suggestion.
Let's say you're a DRK and you Plunged at one mob while throwing Unmend at another. So you have, for the sake of argument, 100 points of aggro on the first and 400 on the second.
Your team is well-coordinated, and they start attacking the Unmended enemy, leaving the Plunged one alone for now. Unfortunately, the BLM forgot to use Quelling Strikes and their Fire IV pulls the mob to them.
So you now have a mob with 100 aggro that's attacking you, and another with 400 that's attacking the BLM. The "lowest on the hate list" is meaningless since it's attacking you just fine, it's the other one that you need to get more aggro on.