Even accounting for the positive side, a system of measuring performance vs. reward would be oversimplified and arbitrary.

The DRG dealt a great DPS, so he can get a better reward (There's no need to be a "minimum value for reward")
There would have to be a minimum value of some kind. What distinguishes "great" DPS from "not-so-great" DPS? How else could the game possibly read and rate DPS performance?

The BLM also deat a lot of damage without being first on hate list, so he should be rewarded too.
What if the strategy depends on the BLM, or multiple BLMs, taking hate? What if the BLM is next on the hate list when the tank drops dead or disconnects (deliberately or not)? No system is dynamic enough to keep up with players' every strategy or eventuality.

The BRD buffed the party accordingly (Ballad on mages, Paeon on DD, etc...) so he should be rewarded too.
Again, no system is dynamic enough to know what players want, and it would be silly if it imposed a standard of "buffing accordingly." Does the system know when the DD could benefit more from Paeon or Minuet? What would it use to determine that? Hit rate? WS frequency?

And I can't argue with PLD.

Look, I wish we could have material reward for our individual performance. I'd revel in it. But without a thousand GMs monitoring every piece of content and reading chat logs 24/7, we will never get a system more sophisticated than a parser. You know parsers can't measure subjective data on "knowing your job" or "playing better," so people would be unfairly judged for doing something clever and breaking routine despite a successful event.

There is simply no way for the game to read and understand anything more complex than parser data. That's why I say, let the game only reward successes of the party collectively. No arbitrary reward system for the treasure coffers and no arbitrary reward system for the individual.