I have done all three roles, I am of the mindset that a group should always pull based on what the party is capable of and not what one or two people are capable of. 1. A healer is not struggling to heal. 2. Dps are killing mobs in a timely manner. 3. Tank is not getting their face smashed in and is holding solid enimity on all targets. These are the 3 signs of a good solid group if a group is failing at any of these points or all three there is a problem.