Only time I had healers forget to heal at all was one specific fight during a DF run of Syrcus Tower. Was tanking the 2nd boss as DRK and the 2 WHM i had didn't even bother throwing a single at me as I was slowly dropping health. Not even when i was at 10% health. Not even a Benediction.

And while it was only once, I have to admit it felt really awful when it happened. Specially with it being a slow death.

To me, a Healer that doesn't DPS during downtime is a bad healer, and I can somewhat bear to call them out on that during a run. A healer that doesn't bother throwing a single heal is not a healer and, as such, it doesn't belong in that spot. Thus, it's a kick for them if they don't bother with keeping people alive repeatedly. Thankfully, I have yet to get to that point (though it's mostly because I usually queue DF as a Healer).