The META (Most Effective Tactic Avaliable) is made by pure math. There will always be one team comp that is the best without any buffs or nerfs happening that composition won't change. Not only that but the meta party is far more significant than equiping a said BiS.

Guy just played DRK because he liked it and the team was ok with it. While it can shorten up the clear chance a bit, it won't do it by much if the team is skilled enough. Now on the BLM side, everything else you remove for it makes you lose an extremely powerful ability for progression and in exchange you get nothing but a job that will either die surprised by mechanics you don't know or low damage to play safe and not die for those said mechanics.