95%. 5%. 1/5th of the time? Where are these numbers coming from?
With some rare exceptions, almost all raidwide damage is magical. Delirium mitigates that. There's not a single relevant fight in the game right now that has no magical damage except for A2S where DRK pulls its weight in AoE DPS alone.
The whole DRK is useless b/c MNK exists is a very poor argument. Most people that actually care about clearing content also care about optimal party composition, so 9/10 in a fight that matters, they won't be in the same static.
If I meet a MNK in the DF, and if any DRK does, for that matter, take advantage of it. Let them worry about the -INT, drop Grit and rotate CDs, enjoy Blood Price and Blood Weapon combined and spam DA Souleater for even better DPS, only ever using Delirium when you get really low on MP. This is what I do and it works, you do a crapton of damage. Win-Win.