It's not about generating enough threat. Sure you can do your single target rotation on pulls after a couple flashes but that's slower dps overall. Seeing as there's 2 main goals in any encounter (kill stuff, not get killed) doing both of those more efficiently is very valuable. PLD dps on trash pulls is terrible, and it really doesn't do any better at not getting killed than the other 2 tanks.
Why give them a crossclass which doesn't even make sense when their own native skill should have damage the way DRK's does? And then call that the more basic solution? It's easily more convoluted in several ways, and not more basic in any. The only problem with adding damage to flash would be in other classes setting it as a secondary (particularly OP on WAR) but PLD already has a trait that changes their flash, so it's literally just tagging 1 more thing onto that trait, rather than all this other stuff to give them an awkward skill they have no business having. Might as well suggest they get Unleash as a crossclass, saying they should get Overpower.
edit: not to mention making Overpower a crossclass would give it to DRK too and DRK with Unleash+Overpower would be pretty sick.