The reason Rackham does this is most of their arists work in Fimo which does not handel vulcanization well. So they need to RTV it and cast a urethane master to vulcanize. You are still going to need to use the low temp silicone rubbers that are now available. I beleive GW does this a lot too.
As for US casters, I know the guys over at Fortress Figures have this method working well for them.