Generally, people are very limited in thinking on their own, that's why you have these "metas" and streamers/youtubers having their asses kissed and licked.
Monkey sees streams, videos with some classes and check warcraftlogs and see that those classes are at the top, monkey invites only those classes, even though the difference might be even less than 5%.
Monkey can't think on it's own to realize that the extra 1, 2% has little effect for it's +10 or +15 key.
Like if you check WCL right now, for +15 at 80% percentile, Destruction has a score of 92, while Marksman has a score of 94. The difference is minimal .. and warlocks bring great utility with HS, Brez, Interrupt and portal, so there's little reason not to bring one.