That feels like such a tiny thing, though compared to all the low-hanging fruit, so to speak, by which to reduce inventory bloat already?
I mean, just look at Materia, for instance. It's about the most bloated way possible by which to allow players a means of customizing their secondary stats independently of their (un)lock with gear drops.
Or the many items that are just redyes of a former, undyable set. There's no need to have 9 colors of Shisui Bikini with only three tiny variations outside of color. Just have the 3 sets, based on those actual featural differences, each dyable and usable by multiple gear types' jobs.
The so-frequently shared Striking/Scouting or Striking/Maiming or Aiming/Casting or or Scouting/Aiming or Casting/Healing appearances? Should every dungeon be required to have 7 gear sets despite having as few as 3 appearances between them and to make those sets undyable just to try to differentiate them unnecessarily? All just to add to the grind and inventory space required with no increase in appearances available to players?
That said, I can mostly agree with making Phoenix Down usable in more content, though I might advise sharing a cooldown party-wide on its use. So long as hard enrages remain, even if we make MP more of a mechanic, it shouldn't make things too cheese-able.
And if we were to go that route, using them as roulette rewards might make the roulettes seem a tiny bit more attractive, though I have to agree with Post that we'd probably also want to make the option more obvious / visible.