Of course it matters. Can you kill the bosses without it? Yeah, probably.
But that doesn't mean it isn't important. You can wipe during progression to enrage with like 3-5% left, healers/tanks can combine for more than that easy if they're doing their jobs right. Even on my Resto Druid I try to do damage where I can, lots of fights have quite a bit of downtime for healers, in fact all of them do I'd say at some point or another, so there's literally no reason to be sitting around doing nothing when you could do some damage to the boss.