Healers should not heal pets for the simple reason that if they require healing the pet owner is playing wrong and needs to be replaced.
Pets take very little dmg from aoe, and so splash heals will keep them alive perfectly well. They only take large amounts of dmg or death from avoidable boss mechanics which the pet owner should move them from. IF they cant do that, they cant play their class, and the healer should not have to carry the slack from someone being a liability.
Fights are designed around healers healing a set amount of dmg, which assumes 10 characters (in a 10player raid obviously!) and that people avoid most avoidable dmg. If you add healing targets in pets, and let them take avoidable dmg, you are putting the raid at risk by effectively reducing the proportions of healers in the raid.
Any pet owner expecting healers to heal pets is basically saying they cant play properly, and expect someone else to cover for their inability to manage their class mechanics.
---------- Post added 2013-01-22 at 10:25 AM ----------
Not played pet classes for a while, but frost mages could heal with waterbolt.
Also, pets wont be targetted by most boss mechanics anyway so never get specific mechanics, and the pet owner can move them out of aoe effects.