We healers need at least some able enough DPS in our utility for Solo content, but let's put that aside for a second.
A good healer will always prioritize healing and keeping their party healthy, but when they're healthy what else can they do? Most of us don't wanna just stand there, so we'll contribute with some DPS to kill things faster.
At the same time, while keeping DPS in mind a good healer isn't going to let their party die, if someone is about to then yes it's back to healing. However if you have party members who will gladly lead themselves to suicide, it's not exactly the healer's fault.

A good healer knows when to heal and when to dps. A bad healer will normally fail in healing either because they get tunnel vision dps or simply are lazy. A healer that only heals is not a bad healer, but is really just doing the bare minimum so they're not exactly good.