Yeah, I'm not sure about that in endgame, things hit too hard to warrant being out of Grit. You also can't drop Grit to pop Blood Weapon and go back into Grit. Going back into Grit while Blood Weapon is active, deactivates it. It's pointless to MT out of Grit for most things anyway. You might be doing more DPS but your causing your healer to have to focus you more. I run with an AST most of the time and they have pretty damn respectable DPS also. The trade-off isn't worth it for most things post-60.
Try MT Ravana EX and eat one of it's tank-busters in Darkside alone. Have fun restarting the instance if you didn't pop considerable CDs i.e. all of them. That thing can hit by 10-12k mitigated with Shadowskin in Grit stance.
He's still right, though. DA C&S is huge DPS and should always be used with DA. Dark Knights have plenty of MP regeneration tools at their disposal. If you find yourself hurting for MP, you either aren't using your tools or you are going a little spam-happy with Dark Arts or maybe Unleashes in big pulls. But Blood Price/Syphon Strike should be sufficient to keep you topped in either situation and you shouldn't really ever need to blow a C&S for 800 MP. Not when you could be critting it for over 3k...higher if your gear is better than mine.
Just a tip: If you begin your pull with a Dark Arts Power Slash combo you will literally have enough enmity for a large amount of full encounters. DK enmity combo is enormous compared to even a butcher's block, since we have to play the MP management game. After that first combo you should not be using your Power Slash 1-2-3 any longer (or at least for 2-3 mins; if the encounter is long you will need to refresh it), but you need to open with it to cement your hate because good DPS can easily creep back up if you don't Dark Arts it.
You can do either combo that branches from Syphon Strike after that exclusively and you really shouldn't ever be running into MP issues. Even with big pulls on trash mobs, pop Blood Price. Most of the time you are getting more MP than you can use. Spam Unleash, throw in a good aligned DA/Dark Passenger, drop Salted Earth. You should have hate for days and full MP by the time Blood Price runs out. Coordinate with your healer though!! If you have a WHM and they pop in to begin a big pull with Holy spam, you just wasted a Blood price because the mobs will be stunned. If you blow CD's while Blood Price is up, then you are nerfing your own MP regen but that also puts stress on the healer. If there are a lot of mobs, I will usually pair Dark Dance with Blood Price since Parrying something still causes you to actually get hit by it. If there are 4+ mobs then even with that active you are normally still taking enough damage to keep your MP up.
So I do agree, and the way he puts it is right. Never probably is the right word to use. If you are doing your job correctly you shouldn't ever have to waste a C&S outside of Dark Arts.