What do you think arena rating is?
No, raider io tells people what experience they have. It does not make you shit, if you don't do m+. There are plenty 10/10M who never go beyond 1.3k score. It would be cool if there was in-game combat analyzer for very basic stuff (downtime/missed GCDs, time spent with cooldowns ready, avoidable damage taken).
Built in rio ladder would be nice, instead of having to update addon and rely on 3rd party ranking system while you could have your "arena rating" but for PvE.
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I mean.. I don't think FotM is required until you start doing 22+. At which point you are equivalent of top 100 mythic raider. At that point you play what is the best because it's your goal to go what is beyond. Trust me, i'm a paladin doing 20s now, my class suck at tanking currently, I really don't expect going above 22s and I am actively looking for not FotM players to fill my groups in, because FotM players, while mostly are good doing dmg, but they are far more prone to sucking at using utility. A good druid or a good fire mage is not only "I go booooom" and it is really really significant. I did a +19 DoS the other day with survival hunter. Most limiting factors for me are - we need quick CR and BL. also my healer is melee, so I can only take another melee. Those are the things that suck. You would certainly shake meta a quite a bit if all healers had CR and few more classes could get BL effect added or drums un-nerfed.