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

[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.