Bosses from warzones 1,2 and 3 are different from bosses from Pawn and 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 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 access, especially on new servers.
About warzones 4,5 and 6, as Longshoot said, the double loot didn't affect reward bags.