I'm not sure if Thorim is THAT necessary(though admittedly Mimiron and Hodir are definitely out for 1 light)
if what I've read on the adds is true, they do less damage as their health is reduced, so if we lose Thorim (and keep Freya to avoid people going insane from Yogg being gay with skulls placement in the brain room or him focusing on one person only) then the change to our tactic would be to have either me or Khrash taunt the add that is marked to be healed far away from the rest of the adds/Yogg(
http://www.wowhead.com/?spell=64468)
http://www.wowhead.com/?spell=64486#comments the comment here include a basic, but helpful, macro for the person on the job
though I guess if we can survive to phase 3 without risking it, then Thorim is probably better