Daka is actually a decent healer when she's maxed. With 12.5k health she's actually the health-iest of my 5 starters. Especially since healing is based on a percentage of her max health she actually does decently.
Will she ever compete with Lumi or JC as your top healer? Well no. But for those DS missions she's the best you'll get.
But as others have said, her big sell are her stun and revive powers. And you can't play the hardest missions with only one healer anyway, so yes you need Talia too.