OP, if you are a very good healer it means most likely you meet healers that are not as good as you are. It might also be you just think you are better than the ones you meet :-), well, if you spend a lot of your time curing and raising the other healer, you are better than him for sure.

About playing as if you were the only healer with PUGs, really you are acting in a situation in which the other healer is already doing things, so you won't cure/raise somebody who does not need it if he has already been cured/raised. The problem is doing the same thing at the same time than the other healer and wasting time and mp. But with no previous coordination that is a risk you take. I agree I wouldn't play expecting the other healer to do this or that, you just adapt to what is happening, the other healer also adapts, and usually it goes well. If you see the other healer is in Cleric Stance and using Holy against a big group of mobs, you don't do that and focus on monitoring the group and heal the ones who need it, the other healer included, for example, or viceversa.