Goal of a raid on progress is to kill bosses. And if you progress super fast it's never an issue. If you take 2-3 months to clear the raid though, the first few bosses soon become farm. They are bosses the raid has already killed and is just spending time killing again. At that point, you very much are just killing them for the loot you need to kill the ones you are still progressing on. The moment that loot is useless for almost everyone in the raid except that one rogue who still needs a trinket, you have drama. Especially if skips are available.