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1). Yes, double attack experiences decreasing returns. As does Haste. As does Dual-Wield. As does nearly every stat in the game.
"Double Attack has decreasing returns" is not an argument against it, it's an acknowledgement. You are still getting a positive return that is unique to the job.
It may not be flashy but it's still an increase. If that had been a piece of equipment people would be paying millions for it.
2) 36 MP is cheap and negligible.