There's a difference between mediocre and not-worth-casting.

Stoneskin is mediocre (aside from idle time during a fight, like Garuda teleport [add-phases], titan after jump, ifrit after jump). It has it's uses but it's hardly game breaking. Aside from pre-empting damage, which is not so easy because the animation-complete-crap, a pld is try to contribute to the kill.

Cure is not-worth-casting. You are better off using the GCD to contribute to damage than to try to cast a cure upon yourself or anyone.

I've saved a few Garudas because of struggling healers, by spamming stoneskin. If cure were equipped instead, I wouldn't even bother casting it.