When should I use Dark Passenger?
Use DP whenever it is on cooldown and you have 2 or more enemies, even if it is not paired with Dark Arts. No matter how you slice it, it is more powerful than if you were to use that same amount of mana on a DA/SE single target.

it seems to me that Carve and Spit is one of those things that if you're MT you should save for when you get low on MP and need more right away because you don't have the enmity to keep doing Syphon Strike combos.
S&C is amazing. I thought it would be a DPS move, but 99% of the time, I'm using it for MP. I'm NEVER holding on to it. Trust me, you can never have too much MP

This will probably change in endgame as people start knowing how to completely unload upon the first pull but that just makes DA + PS even more worth as an opener to me.
The DA/PS opener is still that way in the endgame. I'm about i155 now and doing the expert dungeons and it still holds true.