Well, like I said, one of the Gladiators was an idiot and didn't want to use a shield, so that wasn't helping.
But I think the definition of "needing too much healing" is just based on if you run out of MP before your MP restore ability is up.
Healing "too much" was the only way we could get SP at all - that's why the Sea Hare's "Jizzsprinkler" attack was the mages' favorite thing in the world.
I think the randomness is aimed at sort of giving the effect of XI's campaign exp. system, but maybe they just need to double or triple the chance of it happening while halving or thirding the SP gain from time it procs.
And, as much as I was enjoying it, I think AoE individual actions need to give SP once, rather than have a chance of giving it each time it lands. This is why Jizzsprinkler was so awesome - the mages could frequently get 300 SP just from a single AoE heal.