Identical in exact form? Yes, that'd be bad. Just as it'd be bad for their capped and variable forms of contribution (healing, buffing, situational utility) to be the same, or even to share the same exact portions of their cognitive load, total contribution, etc.
Identical in degree/complexity? No, that'd be fine. I do not think it'd be a problem for their (r)DPS kits, especially, to have the same skill ceilings. There, higher is better; it just needs to not sacrifice the space between skill floor and skill ceiling (i.e., it's about equally important that it gets to a satisfying point quickly / is easy enough to pick up) or otherwise end up overly convoluted, incongruent, incohesive, athematic, etc.
If you don't want playing a more difficult job to simply be 'griefing" and thereby pushed out of even the most basic spaces it, especially, would need for learning, it at least needs some reason to be played beyond just personal taste.Also, there's zero reason a Job that "does far less work" can't have "equal optimized value". The only reason not to do that is for people that want to do "more work" AND be given more for doing so. That is, not people who "are bored" or "want to be engaged". They're people who want "to do more damage". That's a different argument entirely.
Most people's personal taste, after all, will include not being saddled with a job that has a harder time doing the same thing... for no possible advantage.
Okay, let's break this down.There's no reason we NEED all 4 healers to have 6 button DPS rotations. None.
You have a limited amount of sustain that can be useful brought. There is no limit, on the other hand, to damage that can be brought, because every fight in this game is ended by damage. (It is the only long-term form of contribution.)
Now, you can split either in any of various ways.
You can offer sustain through suppressing an enemy, fortifying an ally, applying a buff that will reverse damage taken, applying movement speed enough to avoid otherwise unavoidable damage... or even simply healing.
You can offer damage through making an enemy more vulnerable, bolstering an ally, dealing damage (instantly, over time, whatever it may be)... or even --so long as there's a sort of rate of exchange between your damage and sustain, which requires also having those more direct damage tools-- through limited opportunities by which a timely sustain ability can allow for an ally to "cheese" by making their damage higher than the opportunity cost to support that extra uptime.
You need enough sustain (or, capped / short-term) tools that they complement --rather than overwhelm or disconnect from-- each other. But you also need enough damage (or, uncapped / long-term contribution) tools that the sustain tools have further real trade-off vs. the party's larger economy of throughputs and for downtime not to feel unengaging.Now, why can that last bit --timely sustain abilities as a form of indirect damage-contribution-- so rarely work anymore? Because we've sapped so many of the dangers away and fettered so much of healers' HPS to timers that are also easily executed upon (oGCD) and therefore have no competing dynamics (unlike when, say, nearing the time to refresh our DoTs would affect the opportunity cost of healing in that given GCD). Alas, who knew? It's only been often mentioned since Stormblood-onward.
The latter becomes the far more important, though, when a game refuses to push its players (thus forcing our relative healing requirements absurdly low and our uptime almost entirely towards long-term tasks).
How many each among direct damage GCDs vs. buff GCDs vs. debuff GCDs, etc., though? It does not matter. There's plenty of variation possible while still meeting reasonable minimums for skill-interaction.
Each job having a satisfying number of damage-contributing actions would not force them into homogeneity. They're so homogenous right now in part because their sustain kits are so bloated and all else so stripped bare that there's scarcely any room for diversity.
We can't avoid having a basic filler attack, for instance, and the smaller the kit is, the larger a portion of that kit will then have to be identical.