I used to have a similar mindset. "If DPS are getting hit by avoidable attacks, I don't need to prioritize healing them. Just keep them alive."

However, now I am playing Ninja a lot and I changed my mind.

Sometimes DPS take damage when a tank turns a mob that has a cleave attack.
I was using Blood for Blood at a boss fight. Tank was turning the boss and I was hit by a cleave.
Another time, tank didn't get enough hate on adds, but I am attacking adds so they don't go after the healer.
With Blood for Blood active, DPS can take a lot of damage.

Just today and yesterday I had healers who let my HP stay at less than 10% until the fight was over. They made no attempt to heal.
DPSing as a healer is great, but keeping people alive is a priority.

Sometimes "avoidable" damage is very difficult to avoid. AoE telegraphs are generally easy, but a cleave and adds can be hard to manage if a tank isn't handling them correctly.