I'm a verrry new healer (like maybe a month into healing new). Part of it was for glam, other part is that I tank ALL the time and I want to see what it's like on the other foot and help with my focus (since I have problems with it. ADHD is not fun with this game lol.). You should always be dpsing between heals!! It's like boring anyway if you sit there topping everyone off when they're perfectly fine, it's just a waste lol. The majority of people won't say anything so if you're playing wrong you'll never know until you hit the harder raids and stuff. I learned that the hard way.
Anyway as far as HP goes I just go with what I'm comfortable with. Sometimes my reflexes are slow so I'm generally comfy with tanks being at 25-30% before I heal them. As a tank, I get the anxiety but put a little trust in people is what I've learned playing as healer. Overtime I think I've learned it's ok to let some HP go for the dps. When I go back to playing tank I realize most healers won't let me down as long as I'm also doing my job (using mits and my rotation). If I learned anything from the countless WHM videos.. ABCs.. always be casting.Anyways I'm not a healer expert by any means. But I do know the basics and try my best with it.
The person in the screenshots just wasn't dpsing at all. They didn't even have the dps skills in their bar, so they were literally just sitting there and healing.