I came across this from an old forum on theamazonbasin while looking at the Martial Arts Assassin abilities. I think changing this would be useful for an underutilized build and it seems to be just an oversight. Why would these abilities work with negative enemy resists but not % skill damage.
Skills.txt can only store one set of values for physical damage and one set of values for non-physical damage per skill. When a skill applies more than one type of non-physical damage (Phoenix Strike) or has more than one set of values for the same damage type (Fists of Fire, Claws of Thunder and Phoenix Strike), those values have to be stored in Missiles.txt (where the damage can be assigned to separate missiles). +% to Skill Damage is only applied to values stored in Skills.txt, not Missiles.txt. Listed below are the sources of any damage for each charge:
CHARGE FISTS OF FIRE CLAWS OF THUNDER
1 Fists of Fire Claws of Thunder
2 Fists of Fire clawsofthundernova
3 fistsoffirefirewall clawsofthunderbolt
CHARGE BLADES OF ICE PHOENIX STRIKE
1 Blades of Ice royalstrikemeteor
royalstrikemeteorfire
2 Blades of Ice royalstrikechainlightning
3 royalstrikechaosice
With the exception of Phoenix Strike, all charges are applied e.g. releasing 3 charges of level 20 Blades of Ice will apply 231-259 cold damage to the target, 231-259 cold damage to all enemies in a 4 yard radius (including the target) and freeze to all enemies in a 2 yard radius (the fact that +% to Cold Skill Damage isn’t applied to the third charge isn’t actually relevant since it applies no damage).
Supposedly this can be easily fixed:
Missiles.txt actually has an ApplyMastery column, which enables Mastery (+% to Skill Damage) to be applied to a missile’s damage. However, it’s only enabled for vampiremeteorfire and meteorfire (which applies Meteor’s burn damage). If Blizzard wanted to apply +% SD to all elemental charges they could do so (I’ve tested to confirm it), although the fact that they haven’t is more likely to be an oversight than a conscious choice.
-% to Enemy Resistance is applied to all elemental charges, though, so Rainbow Facet Jewels wouldn’t be a complete waste (although a character who always needed +% SD would benefit more, particularly if they could also use the added damage).