Power Word: Shield is in a similar state to Renew. Ideally, you should rarely ever need to cast Power Word: Shield due to its poor healing for Mana spent. That said, it does have a use for periods where you have no other instant-cast abilities and need to save someone on the brink of death. The extra bit of immediate health could be the difference between life and death for them, so sacrificing more total healing and Mana efficiency in that situation can be worthwhile.