Also the random heroic feature will not let your group jinto the instance (says your team is invalid) if you dont have atleast 1 tank, 1 healer, and 1 dps and you can not select a role unless your class is capiable of performing it. For example, a hunter can only select damage dealer, dk tank and/or damage dealer while the hybrids can choose all three

So a 5x dk team for example would never be able to run the heroic daily for frost emblems (as a team anyway)