DPS is the job of DPS.

[HyoMinPark]

In the context that I see, the +20% is a benefit when active, not a 0% penalty when it is not. I do get what you're saying though, and when trying to push tank DPS as much as possible, you are indeed penalized having Grit active. I just have difficulty assigning it the term "penalty"

There will never be/should never be a scenario in which a DRK is using ONLY Grit, and never using Darkside with it, though. There will also never be/should never be a scenario in which a DRK is NOT using Darkside. Especially now that SE removed the MP drain from the ability. The “+20% when active” isn’t entirely relevant because Darkside should always be active. That being said, Grit+Darkside may cancel each other out in terms of loss and gain, but Darkside alone would still be +20%. Grit is still a damage penalty in that sense because it cancels out the gain.

Unless they buffed darkside to +20%, there's still a -5% penalty.

They did buff Darkside up at Stormblood launch. It’s now +20% as opposed to Heavensward’s +15%. :3