What's the reasoning for melee jobs to have three different sets of gear?

[Vidu]

... the Shadowbringer Samurai set is literally body armor. It's not too dissimilar to the new PvP that just released. I'd say that's close enough.

And while yes, that may have been the inspiration for Samurai. The whole aesthetic argument loses merit when Dancer gear—outside their AF set—looks nothing like what a Dancer would wear.

I'm not sure how it does lose merit because of "dancer"-gear, considering that
...outside of the AF-set its not "dancer"-gear but "physical ranged"-gear, meaning that you can have a set thats more tailored to maschinist or bard, giving only one of 3 jobs a really fitting piece of gear
...most of the current gear a dancer can wear was designed and released before dancer was a job, meaning it was designed without dancer in mind
...not even the AF-set is something every dancer would wear - considering that there are many different dancing styles and they outright said they didnt want this "dancing is sexy bellydance and nothing else" image (to bad they didnt actually stick with that when designing the AF-set). Anything that allows for free movement is something a dancer can (and probably would) wear, they're not limited to a bellydancer outfit.

Considering your original argument: What would be won by removing one of those sets and making samurai share with dragoon? At least in terms of design, which seems to be your problem with the dancer-gear: Nothing. We'd just lose one set.
Also: Just because something insipired a concept doesnt mean that everything has to follow said concept afterwards.