Here's my theorycrafting estimate:

You'd probably be OK in a lot of normal instances, but not all. You can essentially go 5xDPS or 4xDPS with a temporary meat shield. Druid could heal some on hunter pets... I think it'd be challenging but fun.

PvP, I'd convert druid to balance to both nuke and HoT the team. Would be fun as well.

Heroics seem like a no-go, since you wouldn't have BOTH a dedicated healer and a dedicated tank. I mean, you could probably pull off 1-2, but I think consistent success would be darn near impossible.