Materia is useful for being able to customize your secondary stats, for example to get to a particular gcd tier or being able to choose between crit-heavy (and therefore variance-heavy) builds or more consistent builds.

It's small changes but it makes enough of a difference for high level play that I think it's worth keeping around.

If I was to make any change I would like to get rid of overmelds, or at least change them from a completely rng-system to something where maybe you have to spend more materia for the overmeld slot, but a consistent amount of materia per slot so people can plan better.