Before level 30 the only healers you have are CNJ and leveled down SCH/WHM/AST, all of whom can slot Protect.
Emphasis on *can*

They shouldn't be expected to have it, particularly because they are better off having DPS skills in those slots at that level.

And you can switch what slots your cross class skills are in at any point. You can slot Protect in one, buff them, and immediately switch it back out and put Cleric Stance in. By the time you get to the first pack and actually need to throw out a heal you'll be good to go.
Too much hassle for something unnecessary, and there's no guarantee I'll be "good to go" by the time fighting starts because I very often haven't been when I've tried fiddling with cross classes during a dungeon.