The reason why PLD's Flash does no damage is because it costs MP, rather than TP.

Making Flash do damage is a bit like making BLM's Flare cost TP instead of MP. You might as well make it cost 0 MP at that point, as it's effectively free dps with no meaningful resource cost. I know that Flash and Unleash seem to be similar superficially, but MP management is absolutely critical when playing DRK, and is absolutely trivial when playing PLD.