And a lot of metal mounts screws into plastic, so for relatively light lenses it's mostly window dressing. Roger at Lensrentals is very adamant about not using metal mounts as proxy for quality.
That's correct, but the definition of "mount" used in the Lensrentals article (I assume that you are referring to the article of December 2013) is:
the mount is the internal part of the lens where the bayonet — the metal part that twists into the camera — attaches by several screws
So in the article it's not the bayonet, which is what
@blackcoffee17 referred to as mount, it's the part behind the bayonet. I am quite sure that a steel or brass bayonet is more robust than a plastic bayonet (at least when using cost-effective plastic materials) given that the geometry is given and doesn't allow to increase material thickness.
That doesn't explain why Canon is using a plastic bayonet on a 24-50 or RF-S 18-150 lens vs. metal on the 15-30, 24-105 (4-7.1) or other light prime lenses. Without knowing all the details one can only speculate, I could imagine the reasons going from expected number of mating/unmating cycles (where I would expect metal to wear off less than plastic), to marketing reasons like"users who buy the first prime lens after a kit zoom are more likely to do so if the lens looks more professional".
But maybe someone has more insight into this matter