Power Herb is not worth it. Choice Band Freeze Shock can OHKO a lot of things including Ferrothorn but you must be sure that 1. the opponent doesn't have Cloyster or Heatran 2. nothing in the opponent's team can outspeed or use priority. It's good for lategame wallbreaking but that's it.
Still, Black Kyurem is much better than White at taking down Kyogre, Lugia, Ho-oh, Chansey and Blissey. A mixed set doesn't care about Groudon either.