A druid is a decent choice for your main group for a number of reasons. The primary reason is that they can cast Skin of the Drake, which is a line of spells that, when cast on another player, makes them get healed every time they are attacked. Skin of the Drake is great to cast on your puller when they arrive at the group spot. Skin of the Drake was nerfed in early 2015, and is not as powerful as it has been in the past, but is still a beneficial buff to put on your tanking character.
Druids also do decent (but not great) DPS with their epic click and Seismic Shift spell, especially if they have Ultimate Charm. Their group regeneration is very large when coupled with heal modifying items. If you do plan on using a Druid for their AoE DPS capabilities, you should make sure to cast Werewolf Form (self-only mitigation), and invest in Slimy Sapphire of Cowardice for threat reduction, as their PBAoE can easily pull aggro, and they tend to be squishy.
Even if you do not plan to run a Druid in your main group, it wouldn't be a bad idea to level one simply for Spell: Blessing of Ancient Oak
- Spell: Blessing of Ancient Oak (group HP buff)
- Spell: Skin of the Drake (Buff which heals the recipient every time they are attacked. It will scale off the
recipient's charm, not the druid's. Once your tank has an Ultimate Charm, consider this spell a miniature immortality)
- Point-Blank AoE damaging epic click (maximum of 4 NPCs)
- Spell: Seismic Shift (Point Blank AoE damaging spell, 16 second recast)
- Spell: Ancient: Druid Regeneration (6 minute group regeneration)
- Spell: Form of the Werewolf Werewolf self-only mitigation spell