Well as a tank I very much like having a balanced druid in the party. 

Good caster aura: all party members within 30 yards have their spell critical chance increased by 5%

Can pop out of moonkin to provide backup heal :) always good for tanks, and has saved my but on tough bosses

Provide some CC; hibernate and cyclone

Solid single target spell dps.

Great armor in moonkin form, can survive nicely on their own.

Plus I'm always happy to see three trees rush in with the other melee dpsers

EDIT: And battle res is sweet as well, fight still going on, and mage is dead?  BR, hello mr mage, nice to see you again.  Innervate mage for mana regen... win the fight

You may like the 5% crit for a raid but for instances DPS should DPS.  Boomkins will be outperformed by mages, locks, rogues and hunters.  I have run instances up through outlands and not needed a dps to switch to heals to save the tank thats what HS and pots and bandages are for.  Much better CC classes as well.  Take DPS instead of hybrid next time and you may watch the boss die quickly and not need a 2nd healer.