You pretty much answered your own question.Halone as I believe it can be reworked from being a highly situational combo to a bread and butter combo of both classes. I know some of these concerns have been brought up before but I'd like to bring focus to this particular combo.
This combo is pretty situational, and thus...working as intended.
The defense down debuff is pretty powerful, does decent damage, and when combo'd is crazy.
I wouldn't mind the cooldown being lowered a bit, but the mechanics of it are fine.
Flat Blade and Spirits are the bread and butter. Easymode Rage would be way too much icing on the cake.
The only changes I know of that PLD may need, but at the same time would make it way OP is the ability to Tank Multiple Mobs, as well as War.While I know where the OP is coming from and agree with the notion of PLD needing changes, I don't think that Riot Blade was meant to be bread and butter in a DPS rotation. The cooldowns are too restrictive.
What it does good right now:
1) Can block 50%+ of physical attacks on Bosses
2) Damage reduction with Sentinel
3) Highest single target Enmity Generation
4) Medium Damge to High Damage potential
5) Self Heals/Party Buffs/Cover
6) Enemy Debuffs (Goring Blade, Defense Down. Aero, Stone)