It's situational really... I mean, in low level dungeons, if the tank isn't doing huge pulls, I can literally stay in cleric stance and through the dungeon. They take little damage, my 150 heals are plenty when his hp bars are around 1000. Now I've always been an advocate of maximizing yourself in what you do. For me, I always try to stance dance with my healer, as I do my tank as well. When there is opportunity, I will push out more where I can. However, by no means am I a flawless healer. I've accidentally spam clicked cleric stance in panic mode when a tank takes an AoE hit from a boss and I'm in cleric stance already (and then end up taking it off and PUTTING IT BACK ON from my spam clicking). There are times when I'm simply in a panic because I tried to do too much DPS as well and let someone die. And I'm still not great at the whole "pay attention to status effects" part of healing.
I've always had the mentality of "every bit helps". If you don't have that mindset, why bother having a PLD tank attack? Just have him flash away endlessly and sit there popping defense CDs. His attacks will barely help out (sarcasm). It's kind of like sword oath in general. Everyone believes it to be very underwhelming because it's not a lot of extra dmg output per hit. But it adds up quickly over time. Very quickly. Just like mobs hit for very little, when you group up a lot of them, guess what?! That "little damage" you weren't worried about all of a sudden becomes a big deal. Why put on a protect? It barely gives any mitigation?! Why do stoneskin?! It only stops 10% of your hp worth of dmg? Why put on any DoTs, it barely does any dmg?! Because it adds up. Everything helps. If you're going to have the mindset of "that ___ does so little, why bother?" then why not just say, "if I can't 1 hit any enemy, whats the point?" What's the cut off for how much damage is worthy in your eyes to be valuable?
I understand that sometimes a healer needs time to recover his MP, that is fine. Being conservative is fine. Knowing what is do-able and what is not is something every healer must gauge for themselves. I will never condemn a healer for helping to DPS where he can. If he makes a mistake once, I can accept it. If he wipes us 3x times, then yea, I might chime in with something along the lines of less DPS, more healing. Everyone has a primary role to play and I hope they make it their priority.
Anyone would be mad at a tank not holding aggro on a mob.
Anyone would be mad at a healer not keeping players alive.
Anyone would be mad at a DPS not killing things.