Their original post implied they were turning Grit off to use Blood Weapon on 4+ mobs, so I directed them to an appropriate scenario to keep Grit on (since they are less comfortable with having it off) and still do more damage/speed up the run for the party. When you drop Grit, most of the time, you're doing it for BW primarily, you're not doing it to make your AoEs stronger unless in some rare scenarios (OT in solo tank A2 for instance). BW doesn't synergize with your AoE. If you're dropping Grit for only a short time on 4 mobs its likely you're doing it for BW and not for AoE unless you wants to be manafloored very quickly.
The point is, if you're disabling Grit against 4ish mobs, to AoE/self heal, you're not able to do it as constantly, liberally, our repeatedly as if you were tanking more and getting hit more, which, as it happens, is easier with Grit on.
Of course anything you do in Grit will do less damage than its out-of-Grit counterpart. But DRK as a tank craves tanking huge packs, the bigger the pull/more mobs tanked that's more AoE DPS, more MP, and more self-healing. It also means Grit is less of an option and more of a requirement due to the sheer quantity of incoming damage. There are pulls I do in the Arboretum and PSHM that I would never in a million years do with Grit off, even at the end of the pull, popping Shadow Wall my HP plummets until I DA that first AD (I'm actually solo-healing myself for the first couple GCDs).
I'll advocate having Grit off all day for bosses. For speed-run dungeon pulls (which DRK is built for) not so much. My playstyle is to keep Grit on during dungeon trash sections and just pull as much as my body/the healer can take, AoE/self-heal/BP like crazy, DADP the whole lot of them and DADD to go full dodgelord and then single target until BP comes off cooldown, then rinse-repeat.
I'm sure you can keep Grit off if you pull less but this also makes your AoEing less effective overall (less targets) and this lessens potential self-healing and potential BP returns because you just can't pull as much with Grit off as with Grit on. Its just not in the numbers. And from what I've come to understand about the DRK job this is a way to maximize dungeon runs and AoE scenarios and is an area where Grit actually shines. The job craves big pulls, that's just the way it feels, and the more you pull the more bang you feel like you're getting for the job's buck if you will. More mobs just fuels the fire and because DRK's AoEs utilize an endlessly renewable resource (read: not TP) it can AoE longer with MP to spare and then do it again 25 seconds later without skipping a beat.
Its just a quality of life utility that DRK brings to 4-man content or trash-heavy 8-man content, and it just so happens to exponentially synergize with staying in Grit.