Only restores 1 slot chosen at random.
Celes is actually really useful without Runic purely because her SB stacks with all other types of mitigation. A 50% RES boost that stacks with Shell/Wall is pretty damn good. She also has really high natural RES which is useful for her survivability (granted her HP pool isn't great)
It's a shame she can't equip Lifesiphon or the Knight abilities though.
I think the reason she doesn't see as much use as she should is that she has to compete with like Lightning, Bartz, Cloud, Squall and Steiner. And they all can do Spellblade + something else + some have 80 cap unlocked.