It's not useless because it's not about trustworthiness but rather about expected functionality. I may consider google.com and amazon.com as trustworthy in general, but the user does not want anything from these sites that needs WebGL functionality, so if they try to enable WebGL, that doesn't serve a user need and needs to be denied; and shadyjoessuperfun3dgame.com.aaaaaaaaa.ru might be less trustworthy, but if someone wants to play that superfun3dgame and it needs webgl, they might reasonably enable webgl and the related tracking opportunities for that site alone - but not 99% of the rest of the web.

It's more about the general paradigm of recognizing that yes, there are some web apps that would need extensive functionality that might be abused (e.g. advanced copy/pasting features for an online document editing tool, or the ability to connect to USB devices, or camera/microphone functionality, etc), but also that most web browsing does not involve things that user desires to be a web app, and all those sites should be limited to a quite narrow sandbox of functionality. All these fancy features with privacy risk that are needed for some advanced web app replacing desktop features are fine and useful, but they should be exceptions on a per-site basis, not enabled for the whole browser so that they can get used by random blog that includes a standard ad-network javascript file.