So Ragnarok Darks make better Sub tanks than Apoc darks. Because having a 10% Gear Haste Aftermath and Drain on WS doesn't make a difference. Cause You would rather have WHMs cure bombing, running low on MP, and pulling hate. Did I get that right? Let me know if I misunderstood what you are saying.
You've made quite a few leaps and assumptions. Mostly that show a lack of understanding of the event.
1. The difference in damage between an Apoc and a Rag in Legion is not going to be made up by the drain on WS. In fact, the drain on WS is not going to be enough to keep you up anyway, so you will still need a WHM curing you.
2. With the significantly lower damage output, the mob will stay up longer, thus requiring the WHM to cure everyone longer, which is more likely to cause mp issues than not having the intermittent, unreliable means of draining HP with your WS.
3. 10% gear haste? This isn't level 75. Capping haste via gear while still capping acc, attack, and adding in QA/TA/QA gear is not even remotely a problem at this stage of the game.
4. Any group worth anything will have vile elixirs for their mages on the off chance that ***hits the fan and MP somehow gets too low, although this is a lot more likely to be related to issues with stunners than what weapon a DD is using.
I've personally witnessed the same DRK regularly come with his 99 apoc, then finish his Rag, and go from parsing
as low as 7% to being on par with the usual suspects for winning the parses (Not that parses mean everything, but just an example of the difference between the 2 weapons in the same player's hands). Kill speed is important in an event like this, in order to minimize the amount of time your stunners have to stun TP moves, letting BRD/COR buffs stay up, keep healer MP up, relieve your PLD of holding as many mobs, and maximize the number of drops you're getting by killing more iterations of the final wave.