Back to the real subject.
I think Mist is pretty good as a healer and when she promotes I rarely used her to fight (but on my next playthrough I'm using the magic swords) but the horse helped her a lot since (I only equipped her with an iron sword) she could heal and run instead of me having to waste a few units to guard her. But I think that having more healers is better so I used everyone who I trained and is able to heal (Rhys, Soren, sometimes Ilyana, Mist, Tormod).