Furthermore, this worked so well with Stamina because Gladiator's healing plate gear had large amounts of both spellpower and stamina which turned into even more spellpower. Holy geared prot specced healers had by far the highest spellpower in the game.
The main problems are the mana pool, and the fact retribution Gladiator gear has low strength to allow for stam/resil and also that on some pieces, strength is actually replaced with pure attack power.