This just makes it even more annoying because the rat dies for half you rotation. Everyone loves to parrot the godawful "just use it earlier" argument as if you always have 100% foresight for every scenario in every pull, but even then you still at least have some idea of when you will take damage. You have 0 clue when someone else is going to take damage unless its astoundingly obvious. The times when you need to be able to shield someone else are completely random and you will constantly run afoul of the rat being dead preventing you from being able to cast the shield on the person that needs it.

If anything this just goes to show the rat shouldn't be casting the shield in the first place. Get it rid of it like they got rid of searing light and just let the Summoner use it whenever they want.