You are factoring a full pyshical group, something that is not the norm. Usually is 2 casters/2 phys and with the tank damage nerf (not even counting pala that is sometimes casting and not using the buff in that case), you could say that is a relatively minor benefit compared with things like trick atack, without mentioning that it requires a good ammount of coordination to get the best of the 4 seconds of 10% damage