They've been jury-rigging mana into DPS roles since forever, and its probably high-time they've replaced it entirely for non-healers. The only specs where it remains any kind of consideration are the three Warlock specs (minus Destro if you take RE) and Arcane; even Frost and Fire Mages have been moved to proc-based rotations.
After twelve years Life Tap is little more than a relic. There are other "Warlocky" resource mechanics that could be used, and more imaginative ones to boot.