I think it's a solid move for PLD to have but it never bothered me a lot. Saving somebody from being obliterated is powerful, but it also comes at certain costs. Executing Guardian on a target without using cover is tantamount to suicide. Executing Guardian on a target and use cover (to enable them to emergency elixir, etc) is potent, but it also spends the Guard on the PLD, which becomes a sitting target once it's faded off. If the PLD is smart, they'll plan ahead and backpedal, but the fact remains they still used their guard for it, which is quite a cost for a tank imo. If the PLD has already burned purify, they're suddenly a potential prime target for crowd control and a kill. Guardian can also throw the PLD into a very dangerous position if the person they're trying to save is exposed right in the middle of the enemy party.

Obviously it's hard to punish or outright impossible if the PLD has full resources, and especially guard and purify ready. They'll probably not be punishable. But sometimes they don't, because they can't control when an ally will need saving.