Don't you think? We already have good mana regen as disc (unlike holy) and chastise would give us at least one reliable way of keeping a dps off of us and buying us some time. Atm discipline priests are so dependent on others to survive it's very frustrating. No other healing class is as dependent as us. Without help from others we can get completely shut down even by a single dps. We can't slow people, we can't run away. We only have fear, which is on a long cd and is not good enough.
Shamans have totems, frost shock, can kite in wolf form and hex.
Paladins have plate and HoJ and can tank dmg pretty well.
Druids can kite + have cyclone.
Holy priest can dispel poison and run reasonably fast after bubbling, but if anything other than a poison slows us we have the same mobility issues as disc priests. But sometimes chastise gives you time to maybe mind control a dps or to cast a heal on yourself.
Discipline priest are supposed to be a bit more offensive by design, but atm holy is a lot more offensive oriented than disc just cos it has an extra CC, chastise.