This is an amazing guide to get the big picture of how weaken is useful.
But I definitely did personal testing and it seems that Weaken is so much more powerful especially when it happens 100%+ of the time and it practically makes enemies defenseless against the build elements. Weaken high enough with ascendant as another option and exponential DMG like Frozen or so is a huge deal after all. Exponential buff in DMG but on almost ×2 the scale.
Without any ignore resist, weaken nor effective, having Frozen explosions from Frozen set would lose DMG instead of gain due to enemy resists and immunities but they get rendered basically useless after such high weaken.
My hypothesis of enemies having -% resist from weaken was correct.