Honestly niche is how often will you actually use something.
Definition: niche is a specific portion of a market that is united by a common interest or demographic.
Phalanx's Niche is reducing damage taken.
MNK with cure potency,
Can monk natively cure? No.
Does Monk use healing subjobs to utilize the Stat often? No,would be rarely at best.
Am I regularly in situations that this will benefit me? No.
This equals niche.
18 different accuracy sets,
Do I need every tier of accuracy mapped out? No.
Will I use these sets often? No.
Am I regularly in situations that this will benefit me? No.
This is niche.
BLU with Phalanx.
Can I natively cast Phalanx? No.
Do I regularly sub RDM? (Your answer here)
Am I regularly in situations that this will benefit me? (your answer here)
If you answered yes to one or both of those questions, it's not niche, because it would:
A: be something you have access to that you should use even without equipment enhancing it. At 144 enhancing skill(49RDM) Phalanx is a flat -12 damage, until you're getting hit for 200 damage+ per hit phalanx is reducing more damage with 0 gear, including enhancing skill+.
And
B: Is easily enhance-able to some ridiculous levels for a sub job ability. Merits(16), enhancing neck(10) brings the DR up to 15/hit. In 2 easily acquired steps, the fact that Taeon and her can double that easily too is another obvious thing.
Yes, if you rarely use /RDM or never have lots of things hitting you for low(sub 200) amounts of damage, Phalanx+/Enhancing skill looks niche af, but if you'd give it a second look it's one of the most useful spells that most people could have access to.
And since we're on the Rune page, talking about Herc/Taeon gear, it's not optional, it's necessary for the job's page we're on, As a tank reducing damage is your primary focus. And if you play this job and have the gear, every other job that can use it, and has access to phalanx, should be using it because it's free Damage Reduction.