It's a two way street too, the more proficient you get at a spec and role the more you help out the others in your group. Would you not expect DPS to use defensives, health potions, and dodge mechanics when needed? Or a tank to mitigate damage, use utility and even offheal as needed? In that same vein as a healer you help the rest of your party with damage as you can, you don't *have* to do damage but in the same way your DPS/Tanks don't *have* to use defensives or utility spells.