long story short:
back on cata -> int = healing power and mana; spirit = mana regeneration;
now on MoP-> int = healing power; spirit = mana regeneration;
see, healers were double dipping in mana on gear, and the devs had to balance mana pools around it. that leads to 2 aweful effects: insufficient mana at low level gear, and infinite mana at high level gear. the first one is frustrating, while the later is OP.