(disclaimer, I dont focus on the yellow traitline too much in this post due to me feeling like its not in a very strong spot right now, hopefully with the new class updates it'll get better! I do however use some yellow traits regardless of if I'm redline or blueline specced so I know it has potential)
I see people all the time saying a captains main role is a healer.
Our main role is a support, we have multiple uses outside of the blue line. A truly good captain who knows about each traitline(not very many captains do these days) can dps just as well as he can heal, I've been playing my captain for 4 years now and I've gone solo, group, raid. I've gone thru blue line phases, I've gone thru red line phases and I've tried out yellow line multiple times. I decided to practice both healing and damage for a week and take notes on the numbers I saw, shockingly..the numbers were nearly identical. Oh wait thats not very shocking..and let me tell you why,
Captains weren't designed solely to heal, nor were they designed solely to dps or tank, we have multiple traitlines that we can use to fill each of those roles equally, or mix them and be a hybrid of dps and heals., or heals and tank(other combos don't work that great from my experience.)
We're the support class! The blue traitline is just as good for healing as the red one is for dps, saying captains cant dps might as well be saying they cant heal because a good captain can do both equally well.
Captains aren't healers, get that silly idea out of your head, we're support.