Some reasons why people may reject the notion that "System/Rules matter"

[Manbearcat]

The claim that system matters is not a claim about what people should care about. It's a claim about causal and social processes in gameplay. The people you point to who don't care about system are still having their play influenced by the system they are using. If they adopted different processes of play and/or resolution techniques they would find their play experience changing. The fact that many of them probably don't care to do that just drives home the point that those things matter to the RPG play experience.

Exactly. Here are two procedures for deciding how the PCs acting indirectly upon the setting manifests (say, they accidentally unleash a magical plague on the city sewers while destroying a powerful Sorcerer and his laboratory):

1) <PLAYER/TABLE-FACING> Alright, lets make a Fortune roll to find out the immediate fallout. Ok you knocked over the Sorcerer's alchemical table and all of his beakers crashed to the floor and the combined arcane sludge seeped through the floor drain and into the city sewer systems. The Sorcerer is Tier 3 so 3d6. 1-3 and little to no effect, 4-5 and mixed/partial effect, 6 and great effect, 6x2 and critical effect.

Alright. <rolls on the table> 6! Great effect.

As you watch the Sorcerer's manor house burn to cinders and ash, you hear a raucous and a scream down the street. You see a crowd disperse wildly in a panicked rush but you can't see what they're running from. In a moment though, you get the gist. A nearby sewer grate is thrown from its housing in the cobbled street you stand on. A homeless person (man or women is impossible to tell at this point) slinks upward and outward, skin sloughing strangely off limbs and bones contorting them into impossible shapes. The abomination awkwardly half-lumbers and half-oozes toward you with madness in its disintegrating eyes!

2) <GM-FACING> Ok you knocked over the Sorcerer's alchemical table and all of his beakers crashed to the floor and the combined arcane sludge seeped through the floor drain and into the city sewer systems. <GM EXTRAPOLATES VIA THEIR MENTAL MODEL AND/OR WHATEVER THEY WANT TO HAPPEN>

Alright, x happens!

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Whether you're going from 2 to 1 or from 1 to 2, what is happening in play isn't an illustration of system doesn't matter!