Back home now, so I can look at each of the debuffs and why some of them work for the intention of synergy, and how some do not.
Slashing
This is only given by warriors and ninjas, and utilized by all tanks and ninjas. Disregard the fact that warrior is the off-tank to have (thus the application of slashing resistance is always there), it still has some synergy with ninja, because it allows the latter to exclude dancing edge from their rotation (which is less potency than AE)
Blunt
Is only given by monk and utilized by monk. They could have either given MCH blunt attacks, or even hybrid attacks (certain attacks like blank, gauss round, head graze, turrets, etc deal blunt damage, while other applicable skills like heartbreaker deal piercing, basically to match the animation/effects of said skills becuase not all of them appear piercing, not even clean shot) and MCH gets a skill that has the same debuff as dragon kick, either gauss round or another skill that requires ammounition or something. What this does is allow MCH to buff part of their dps (and not all of it), while giving MNKs the same synergy with MCH that exists with WAR:NIN, it allows them to opt out of using dragon kick which is effectively weaker than bootshine.
Piercing
Given by DRG, utilized by DRG, BRD and MCH. DRG is unique in the case that this debuff is applied as part of their regular rotation. If optimizing their dps was the case, they have absolutely no say in the matter because they have to do disembowel for chaos thrust. Because of that, there's really not much reason to give it to BRD/MCH because it only synergies with each other (and a typical party composition will not have both in the same party)
Magic
Is only given by BRD in the truest sense (hypercharge gives magic vulnerability, which stacks with resistance down). There can be other applications for this...such as dork knight through derilerium. Delirium as it is now, it's practically a joke. It doesn't offer blunt resistance and is effectively a useless skill with a monk in your party. You don't want to make it physical damage down if DRK is supposed to be the magic tank (opposed to PLD being the physical tank). There's not much reason to use delirium even outside of grit because it's a measly 10 extra potency (which then gets eclipsed by Dork Arts + Souleater). It's also the only combo finisher that does not have a dork arts effect, so why not attach a 10% magic resistance (not stackable with foe) down to it's DA effect, considering that DRK is also partially magic damage.
The greatest area of concern is the blunt resistance because it's only ever utilized by one class and has absolutely no synergy, but that als oboils back to MCH being an incredibly wasted potential when tehy could have made it blunt, or even a hybrid that can do blunt resistance down with -Int.