Ok, let’s do some math then.
Let R be relative difference in damage with and without resistance reduction (i.e. R is dmg with reduction over dmg without reduction). Let x be the player’s resistance reduction and y be the monster’s resistance. Then:
R = 1 + x/(1-y)
Indeed, resistance reduction is a multiplier. For a monster with 50% resistance, each point of resistance reduction is equivalent to a 2% increase in total damage. Let’s see how effective resistance reduction is for some value or monster resistance less than 100% (to avoid dealing with singularities). We need a partial derivative of R over x:
dR/dx = 1/(1-y)
Effectiveness of resistance reduction is inversely related to monster resistance. We can plot this relationship to get a better sense of what’s going on.
In a world where the average monster resistance is 80, each point of resistance reduction is 5 times more effective than in a world with no monster resistances. If you want to nerf resistance reduction, while keeping it’s usefulness as a tool to push your damage trough vs specific mobs (bleeding vs skellies etc.), all you have to do is reduce average monster resistances. Simple.
In conclusion… The statement “it’s a multiplier, you always want it” completely misses the point. QED.