[Hamahakki]

The three healing classes are pretty balanced at lower levels, but Druid falls behind Cleric and Shaman as the mobs get stronger.

Cleric heals are much more efficient than Druid and Shaman. It's not that groups can't function with a Druid or Shaman as the main healer but you can't really chain pull, and you'd always be killing more if you had a Cleric. Cleric durability buffs are also much better than Druids (though this doesn't matter much if you don't have a Cleric because you aren't CHing)

Shamans are the only healers with meaningful DPS buffs (much more STR than druids, DEX and HASTE) and the best slows in the game. Slow isn't that good at low levels but they become almost essential at later levels as the slows become stronger, fights last longer, mobs hit harder, and players have more mana. Cannibalize and later Torpor also means they heal better than Druids.

Druids are great at lower levels, but their strengths (DS and strong nukes) aren't as useful in the high level game. Thorns just don't scale enough; a damage shield of 9 at level 20 is MUCH stronger than a shield for 32 at level 60. High-level mobs have too much health, and they also hit less because of slow. The ability for all casters to fill in as a DPS class also diminishes as mobs have too much health and melee buffs become so strong.

Ironically enough, the Druid's best tool for groups at higher levels is just porting. You can collect people from around the world to start a group where it otherwise might not have gotten off the ground. It also makes it much easier to replace people who leave to keep a good group rolling.

Edit: grammar.