the priority of healer is to keep everyone alive and remove debuff
the priority of tank is to hold aggro and minimize dmg taken
you could not say a healer not doing their job for not doing dmg, they could be new to dungeon, they could be less competitive healer, not every healer that refuse to do dmg is lazy, and it is not a must, not in the past, not in present, and probably not in future
a healer doing dmg in dungeon is a good healer, a healer not doing dmg is not a bad healer, just less skilled
you could stay in DPS stance and use STR gear as long as you did your job, to hold aggro and keep the dmg u take low enough for healer to cure up
same story that a tank who could switch stance, while keeping aggro and not taking excessive dmg is a good tank, but a tank who stay in tank stance all the time is not a bad tank, just inexperience or less skilled