Bosses from warzones 1,2 and 3 are different from bosses from Pawn and Hur, Fur, these have a lot more items which they can drop while bosses from warzones 1, 2 and 3 have just few.
In other hand they will drop two of the items used for the acess access to the warzones, so I'd say it worthy to kill these bosses if you and your team are looking for the items for the acess, specially at access, especially on new servers.
About warzones 4,5 and 6, as Longshoot said, the double loot didin't didn't affect reward bags.