That's much like saying that a druid tank doesn't need defensive cooldowns because they have battle res. Or that a moonkin doesn't need good AoE because they can go to cat and swipe.

The utility argument is great, but it sucks when you don't need that utility or you just can't do the job other people can because you've got so many other things going on (most of which are irrelevant). Yes, resto druids have battle res, resto shamans have mana tide. Holy paladins have lay on hands, and priests have brogrip. So what? The important part here is that some healers are better than others at actually doing the role they want to do, and the reason for that is a differential in cooldowns.

We saw the same thing with tanks last expansion: if tanks don't have parity in cooldowns then the tank with the best cooldown wins. We're seeing that more and more with healers - we saw it last expansion with stacking of holy paladins because of sac and aura mastery, and we're seeing it again.

That being said, please don't change barkskin to be castable on others. Having a personal defensive CD is really nice, but more importantly barkskin on a 1-minute CD is just ridiculously powerful when applied to any arbitrary tank. Don't do it. Give them a new ability.