Reduce target's X resistance by 10% (All dps classes)

[Bard]

Haven't done extensive parsing or anything, but Foe's Requiem (-10% elemental resistance) is definitely not a 10% damage increase to a bard's elemental damage abilities. Testing it out on target dummies I'd say it's no more than a 1-2 damage per tick increase for Flaming Arrow and Windbite, if that (really, it wasn't noticeable at all over the normal damage variation per tick and the max damage per tick didn't seem to increase at all). Of course, it's possible that target dummies have no elemental resistance, in which case this form of testing would be flawed.

[Zellata]

Haven't done extensive parsing or anything, but Foe's Requiem (-10% elemental resistance) is definitely not a 10% damage increase to a bard's elemental damage abilities. Testing it out on target dummies I'd say it's no more than a 1-2 damage per tick increase for Flaming Arrow and Windbite, if that (really, it wasn't noticeable at all over the normal damage variation per tick and the max damage per tick didn't seem to increase at all). Of course, it's possible that target dummies have no elemental resistance, in which case this form of testing would be flawed.

Requiem decreases magical resistance, not elemental. As such, it does absolutely nothing for our (bard) skills like flame arrow and windbite - those are considered to be physical skills despite dealing elemental damage. Requiem does however buff all (even non-elemental) magical attacks - i.e. SMN and BLM benefit from it.

[Furon]

Physical resistance is different from magic resistance, just go check your character it will clearly state that your resistance to blunt/pierce/slash is 100 (aka a factor of 1) meaning you take "normal" damage from these weapon types, debuffing someone 10% would lead to that target now only having 90% resistance to that damage type (meaning that damage type will do 10% more damage), same if you add 1 piercing/slash/blunt resistance you would now take 1% less damage from that damage type (because they now have 101 instead of 100 resistance to that damage type

This is apart from a targets armor (certain mobs will take less damage from physical, gargoyle type mobs in AK is a prime example of this.

Magic resistance on the other hand is a % reduction (note that you will have several hundred resistance towards each element), as an example let's say that 500 resistance to fire equals 25% damage resistance to fire, lowering this by 10% (bard song) will only increase the fire damage that target takes by 2.5%

So spell resistance debuffs are weaker on paper (unless we get immune type mobs!) however more people tend to use magic based attacks then there is people in a raid that can take advantage of a physical debuff (monk's crush debuff only works for them as an example) so the overall raid benefit of the resistance debuffs are pretty equal (unless you stack 3 bards and a lancer...

[PessimiStick]

Magic resistance on the other hand is a % reduction (note that you will have several hundred resistance towards each element), as an example let's say that 500 resistance to fire equals 25% damage resistance to fire, lowering this by 10% (bard song) will only increase the fire damage that target takes by 2.5%

This is not how it works in practice. Foe Requiem is more than a 10% damage increase for casters.