You mean hardcore mythic raiding. Though most of my heavy raiding days were back when it was called heroic. But I never belonged to a guild that mandated anything. We took the people who performed the best, and if a fight was better for mages and not hunters and we had a mage on standby we swapped ,outside of that we went with what we had.
It still doesn't answer the question. Why does anyone "need" a BiS list? How are you a part of a hardcore mythic raiding guild and not know which items are an upgrade? Why not just say" We like list because it's easier to know who to give the loot too"? or "this new system makes getting loot too mathy for every piece"?