"We need to get away from this simplistic min-maxing way of thinking"

There is a reason we don't get away from this. It's when you wipe to progression raids with very small amounts of health left - say, anything under 5%. If you've ever had a 5% or lower wipe, and were not able to down that boss in that raid lockout, you quickly will find yourself looking at the people who are lagging in DPS. Once you calculate that a small 3% increase in the lowest 5 players will get you over the hump (assuming no one died to anything stupid), it's really hard to sympathize with them if they play a suboptimal spec, which in many cases can give way more than that small amount.

Are they contributing to the kill? Sure, anyone who does any heals or damage contributes. But on progression content, if they are not maximizing their potential, they could be holding you back and causing you to wipe.

This goes to the whole argument of your 15 bucks a month is outweighed by the 360 bucks a month of the other 24 people in your raid.