Is it your job to DPS, NO But do you want to help the group:
- Finish the dungeon faster?
- Do things instead of standing there and wasting mana?

Then you should DPS, I dont think all the healers have to DPS, but if they are able to and can do it, why not help the group by casting some damage dealing spells? If SE wants healers to only heal, then they would have made mobs hit the players with more damage, which inturn means the healer needs to stay out of Cleric Stance and heal or they could just remove Cleric Stance from dungeons, like what they did with PvP. I think the reason why they have not changed this is, because they want to give the players a handicap, by giving them an extra dps that is disguised as a healer.