It's by no means compulsory otherwise the dungeons would be designed harder and grindier. I keep telling you guys this, that's why FFXIV is the way it is and not a RNG-Damage-Spam zerg-fest. There are people in this thread who think just because a healer isn't healing, they should be damage spamming. It's not the healers role to take up the slack of the DPS.
In the low-level dungeons it feels agonizingly slow to do when the tank is ill-equipped and basically loses half it's HP on every hit. That is reality. If you luck out and get an overgeared tank, they may actually last the 5 seconds required to switch in and out of Cleric stance. Otherwise it's entirely pointless for the healer to DPS because the time spent waiting results in weak heals and slow damage. In Halitali, you know the tank will last longer without heals if you get rid of the adds, which you can do faster than the melee DPS.
It's all situational, but a healer is under no obligation to "help DPS" otherwise the dungeon design would enable it. The only places in any dungeon where the Healer could "help" DPS are when the Boss is invulnerable, and the adds do not attack the players even when they have hate. A healer doesn't need to make up for weak DPS on trash mobs. Figure out if the Tank will last 10 seconds without a heal and then decide if it's worth risking the tank die just to fire off two DPS skills in that 5 seconds. If you know the battle, then you know the answer. Most low-level DF queues you can expect one player to be a write-off, just hope it's not the tank.