This was prompted by two recent conversations I had–first, the one surrounding this tweet, and second, someone telling me about a guildmate of theirs complaining about a different guildmate who was using bad gems for their class.
So, the odd thing about someone using completely wrong gems (or some comparable character setup choice) is that it doesn’t matter, on its face. Gems (especially now) are such a tiny fraction of your overall character strength, and the difference between two secondary stats is such a small fraction of that, that you’re pretty hard-pressed to conclude that someone using the wrong gems has any effect on anything whatsoever. Maybe in some unusually strong cases, that 1 person out of the 20 in your raid will have a 1% shortfall in their output? Even that much is probably rather rare.
Basically, if you imagine that you couldn’t inspect people’s character sheets or stats directly, I’d posit that there’s no experiment you could conduct that would reveal to you whether the people in the raid had optimal gems or slightly suboptimal gems.
So why do they matter? I’ve always only thought there was one reason, and this is including all the way back when I was in a very serious high-level guild and would review applicants. It’s a way of showing that you’ve researched your class. That is something you want to know about an applicant or raid member, and is kind of hard to find out directly.
It’s a little tricky because they don’t necessarily need to be able to know the theorycraft rationale for why that stat is preferred. Just like the gems themselves aren’t the actual point of interest, being able to do/understand a lot of math isn’t the right criterion either. It’s entirely possible for someone to understand the spellcasting decisions, which is the actual important issue, without articulating the theory. But I think what it comes down is, there’s really no way to develop an good practical understanding of the class that won’t, somewhere along the way, involve the information about what stats are best.
Practically speaking, the only way to get solid information on the best play of a class is by reading resources written by others who used some kind of mathematical tool. And those always include information on stats (conceivably they might not include that topic, but that would be rather odd). The only exception is if you’re the person making the tool, in which case you of course can work out the stats for yourself as well. If you’ve gotten all of your class understanding from sources (including “play experience alone”) that don’t contain stat information, then you must not have looked at things which are rigorous. And that actually is something worth selecting for in applicants and guildmates.
So gems turn out to be good signal. It’s not very discriminating, sure. Someone might be using the right ones just by some whim, or by following their Attunement (which is often right), or from seeing other players do it. But I’m of course not saying you should conclude that anyone with the right gems will be a great person to play with (wouldn’t be nice if that worked). I am saying that using wrong ones all but rules out that the person has used any meaningful resources in learning to play their class. And that is very likely to have more than a 1% effect on their performance.