Defiance actually did suck in comparison to Grit and ShO in 3.x because of multiple reasons:
In 3.x Heavy Swing TP cost was 70, and because there wasn't Inner Release for moar Fell Cleaves - which doesn't cost any TP - your overall TP drain was much higher then. Goat was used on melee jobs, their TP drain was usually even higher. So keeping Equilibrium on CD was pretty common. WAR didn't benefit from swaping to Defiance immediately cause of this certain reason.
Second reason was damage penality. In 3.x Defiance damage penality was actually 25% instead of 20%.
Lastly, bloodbath and zerk in Deliverance was more beneficial since you healed yourself more than in Defiance.
Just because SE didn't change anything in this regard and it is working now, doesn't mean it was working then.
Same reason DRK was working in 3.x because of raid design - most tank busters were magical, DRK's DPS was higher than PLD's, and magical attacks couldn't get blocked - and is not working now. There was already an issue with DRK in 3.x, but it just got recognized in 4.x since there aren't many magical attacks (only in O2s) - and they even can get blocked now - and PLD got higher DPS and utility than DRK.
If 3.x raids had more strength based attacks and PLD's DPS was more close to DRK's, PLD would have overshadowed DRK.
Meanwhile WAR got nerfed and buffed from an OP tank - in DPS stance - into a still pretty awesome tank - both stances.
You can't just count Blackblood as a resource for DRK. DRK has the same amount of resources as PLD, yet uses them in different way and is even more dependent. PLD uses its MP for damage - Holy Spirit spam or 'oh shit' Clemency heals, and its gauge for mitigation - Intervention and Sheltron
DRK uses Blackblood for damage dealing abilites, and MP for additional damage and mitigation - TBN or enhanced Dark Mind.
The even bigger difference is DRK's resources are more limited and even more complicated to manage - as they are very punishing if you become greedy. PLD has more tools to manage their resources better and easier.
WAR is pretty forward and simple with just its gauge. It's for both damage and mitigation.
Last, but not least, DRK also pays a high amount of resources (MP) for mitigation - Grit and TBN. As DRK lacks mitigation, esp. prior lvl 70, buffing Grit into an oGCD skill is the least that can be done. Recast time can be ajusted.
Since you don't want to share recast time, I disagree here.
It's just too easy to lose track of your remaining CD time on those abilities once you turn Grit on/off.