True, but what are you going to use instead? Cure? Practically worthless except in only one instance in Praetorium. Protect? No use in a dungeon, and in solo play you can switch to anything you want anyways practically on the fly if you're in town first. Raise? Same thing as Protect really. If you're in combat, can't cast it, if you're not in combat, better for your dead party member to release and run back than have to deal with a 2 minute debuff. Skull Sunder? No combo action to cycle into this, so it ends up being pathetic.
I voted Mercy Stroke because while yes, it's not going to benefit a Paladin as much as a Warrior, the point is it's a skill on a relatively low cooldown to begin with, can be used off GCD, does a little bit of damage, and if you pair it with Fight or Flight + Bloodbath, you'll get healing no matter what happens. I could best describe it as Assassinate on Ninjas. We know it probably won't finish the boss, we know that we likely won't get any healing from it, but we also know that at 20% HP on the boss, all you're looking for is to bring it down just that fraction of a second faster.
That being said, if you run into a situation where ANY cross class skill could be of more use (i.e. Raise, Protect, Cure) then feel free to switch out. I have already mentioned Cure's one real application in Praetorium, but there was one time (only one) where a Paladin's Raise saved the day. It was in Labyrinth, on Atmos, where team A only had 3 members left alive after killing their Atomos, and the only one with a Raise was the Paladin. He raised a healer, and saved the day. One time, only once, have I ever seen the practical application for this skill in a dungeon, but it was pretty bad ass. I'd like to think, however, that it shouldn't have to get to that point, and a skill with much more application across all situations (Mercy Stroke) would be better suited as a Cross Class.