I've seen so many of these threads come and go on this forum throughout the past year, and I find that the most fundamental problem in the debate is a disagreement on the basic premise: is it my Job to ONLY heal/mitigate damage?
Now, the people in the DPS is good camp take major issue with this premise. I find it dubious myself because I think it's an assumption that didn't originate within the game. Therefore, most arguments on this topic the two sides just end up talking past each other: the no-DPS camp insists that anything that isn't healing/mitigating is a waste, and the DPS camp says that decreasing dungeon time is the ultimate goal so standing around is a waste. Because of this, there's hardly ever any resolution to be had and bystanders, who may be new to healing or the game and are looking for answers just leave feeling confused and unsatisfied.
Therefore, in this thread, we are going to ASSUME for the SAKE OF ARGUMENT ONLY that the premise "healers should only heal damage or mitigate damage" is a solid assumption. Spells that increase damage output, whether they are direct damage-dealing spells, or friendly damage increasing buffs like Balance and Fey Wind are optional at best, and a waste of mp at worst. We are also going to assume that overhealing is bad and to be avoided and that wasting mp is bad and to be avoided.
Under these premises, at first glance it looks like healers DPSing is objectively a bad thing. And I think this is where most of the misunderstanding comes from. However, I'm going to prove that that's actually not the case at all.
Take this hypothetical, but in practicality very common scenario, of not needing to heal the tank every GCD. The tank only takes enough damage to warrant using your basic healing spell, say, once every four GCDs. Anything more often than that is overhealing (bad) and a waste of mp (bad).
We now have three options with what to do with our remaining time:
1. Stand there and wait until the tank needs a heal again (approx. 9-12 seconds)
2. Shield the tank with some kind of mitigation (Adloquium, Stoneskin, etc)
3. Cast offensive spells to increase damage output
Obviously, option two is superior to option one. Option three may be equivalent to option one under the given assumptions, but we'll clarify that in a minute.
So with your next GCD you increase the tank's effective health by casting a shield. This, however, only increases the amount of time remaining until you have to heal again: maybe it gave you another two extra GCDs worth of time before you have to cast your next healing spell. Now, that's still objectively a good thing because you've prevented damage, but at this point it becomes inefficient to cast that spell again for a while because shields are pretty mp consumptive and you start running into the overhealing issue again, since the shields don't stack they overwrite the old shield.
So after that you're down to DPS or do nothing. One could argue doing nothing is better, because you're not wasting mp DPSing which would otherwise be used for healing (let's assume MP is actually a limited resource, even though in the vast majority of content it's not in practicality). HOWEVER, there is one reason why DPSing is still superior:
If you are doing damage to the mob, you are killing it faster. If you kill it faster, IT DOES LESS DAMAGE THAT YOU WOULD HAVE OTHERWISE HAD TO HEAL.
This is the fundamental issue that I think most people in the "healing's my only job" camp miss. If the mob does less damage because you cast some offensive spells, it dies anywhere from 15-20 seconds faster. And all that time it's not doing damage. And if it was alive during that time, it would be doing damage which you would be healing anyway. So preempting the damage with mitigation-intended damage spells and healing the damage caused retroactively are in essence the same thing.
Now the DPS also comes with the added bonus of completion of objectives faster. I know that's not an end in an of itself to all healers, but you have to admit that healing/mitigation held constant over the two scenarios (meaning that you've "done your job" equally well whether you chose to do it by dpsing proactively or healing retroactively), a faster run is preferred. Opportunity cost of time and all that.
So there you have it. TL: DR, DPS = mitigation = PART OF YOUR JOB no matter how you slice it. So if you choose not to do it, some people might accuse you of not doing your job properly, and they have an airtight argument.
Final point: if you continue to choose to only use direct heals and mitigation and avoid damage dealing spells, that is your choice and you are within your rights to make it. However, you cannot deny that you are not doing your job as efficiently as if you were using damage spells when healing/shielding is unneccessary. Therefore you cannot expect everyone on the internet to tolerate your playstyle. I would never hope that anyone got kicked from an instance, it's my sincere belief that we can all accept each other's differences for the duration of one dungeon. However, you can't deny that if/when a group takes issue with your gameplay that they do, in fact, have a valid point.