The only occasion I take a PW Champ with my Berzerker is when I field Kharn with them. The reason is that you have to kill the guy with the PF in the enemy unit, before he can strike Kharn and instakill him. Therefore the 5 more PW Attacks at I5 are crucial for Kharn's survival. And Kharn is pretty good against walkers and tanks anyway. The only problem are MCs.