It takes a finite time to cast a heal spell, so what often happens is a heal is wasted because either 2 heal spells are cast at same time or a heal spell is cast just as someones popping a potion.
Dear god no! THis was annoying in FFXI, it would be even worse in WoW when people are constantly using abilities. You're a healer, I know you're going to heal me. I'll trust you to do your job, don't spam me so I can't see my combat numbers.