I started out just saying "it's silly" but that didn't convince them, so I had to break out the hard-and-fast reason why it would be a completely nonsense change.

I'll try to lay this out more clearly, since DrkDays seems to constantly miss the point even when others reword it.

1. Dark Passenger's Mana cost is not a problem - it does not impact your ability to use other cooldowns unless you mismanage your resource.

2. If you change this perfectly-fine Mana cost into a health cost instead, you will do one of two things:
-- Dark Passenger now costs x% of your max health (like maybe 10% or something to make it actually a cost) - and suddenly no Dark Knight worth his salt will ever cast this spell while taking damage from enemies. If you do, you will immediately be labeled dumb and/or shit, because it's absolutely ludicrous to sacrifice your health while a boss or trash mobs is/are wailing on you.
-- Dark Passenger now costs some arbitrarily small amount of health like 1000, as DrkDays suggests. A combination of DP>SE will make you take 0 damage from that attack - in fact, you'd get a net heal. And now you've completely effectively removed the cost from the attack in exchange for 20 potency (since you'll be using SE [260] instead of Del [280] on that GCD). Strict loss for no substantial gain.

3. The entire argument can be summed up as "Dark Passenger's Mana cost makes it very easy to mismanage my resource and have too little Mana remaining when I need it for a skill such as Grit or Dark Arts+Dark Mind." Maybe I'm elitist - go ahead and call me that. But in a discussion about changing the mechanics of a class, you should never consider the situations in which it can be misused and handled improperly - you need to only consider the optimal case. Catering for the people who struggle to maintain BP and DA+SE and DA+CaS rather than those who know how to play the Job is the wrong way to design things.