Look at all those Ret paladins that raided in classic - there are plenty of them despite being the most useless spec for raiding. Then you kinda have to play as they tell you otherwise you just can not play at all They prevent you from playing unless you ”Fall in line” As you head down the skill pyramid, you may not be able to recruit people to fit into your theoretically optimal comp. In practice many will bring 2, sometimes even 3, because if played well they do decent DPS (before SWP only behind warlocks/hunters, and they can beat them on some fights - and this may be exaggerated if bosses die easily and quickly). Obviously you are just farming the content that may be less of an issue, but then if it is on farm, is optimal comp necessary? Realistically I think almost all guilds are going to bring one mage because despite memes of camping them outside for buffs+drinks this is actually kind of annoying to do if you are progressing bosses and so wiping repeatedly. (But prior to that if you think mages have problems…) Melee is also moving up the chain with glaives.
Depending on how hard that proves, guilds which have been running multiple mages may need to step them down for other things (i.e. They fall off a bit in Sunwell Plateau which I think is where the negativity comes in. I guess it depends on how sweaty a guild you want to be in.