To be fair, one group of 8 people is hardly "people are clearing ultimates". It's "people" in the absolute most technical sense of "more than one Human being", but in any realistic sense, it isn't. ONE GROUP doing a thing getting overblown to the point of abject ridiculousness.
NOTE: This isn't me saying encounter design doesn't need changing (I say all the time it does), nor me saying the game doesn't need to require more healing (I say all the time it does), nor me saying Tank and DPS healing and mitigation is fine where it is (I say all the time both need to be reduced) - but the amount of currency in hyperbole people have gotten from the fact one group that INTENSELY focused on a specific fight to figure out how to cheese mechanics with Cover and delayed Raise acceptance along with luck...is abjectly insane. That's like saying "Marathons are easy, a few thousands of people do them all the time", as if everyone was doing it. Or "Anyone can beat the AI Go master, after all, the best Human player in the world played 5 matches against it and won 1, so OBVIOUSLY anyone can do it!"
"But the Ultimate before was cleared with one healer!!" - Yes, also by cheesing things they could, bringing the most versatile Healer Job, having their Tank and DPS heal (like their PLD and RDM were casting Clemency and Vercure), and having to play flawlessly several times in a row until the RNG lined up for them to get the clear. Again, this is not something PEOPLE are doing in a general sense. It again is an argument that encounter designs need changing and Tank/DPS healing and mitigation needs nerfing, but these are exceptions that prove the rule, not rules in themselves.
...sorry. I'm just sick and tired of people seeing one example of a thing from EXTREMELY talented and dedicated people and suddenly deciding it's a game-wide problem when no one else is doing it and the amount of players that even CAN do it is ridiculously small, and most of THEM aren't bothering to try because it's just not worth the trouble. Once one team has done it and proved it's possible, there's literally no incentive for anyone else to try because it's just stupidly inefficient.
People need to stop using those cases as some wider example of reality.
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I agree with your premise, but that example is the dead horse beaten into dust already, and it doesn't even make sense to use it for any rational arguments to begin with.
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OP: I agree in part - Jobs within a role/subrole should not be TOO far apart in what they offer - but not in whole - what you're asking for is bland homogenization. FFXIV's combat system needs to be less rigid and allow for other forms of utility and such before that can seriously be factored in.