I personally don't even recruit Mollok to be honest; he's barely canon at best, and Mordor has enough good heroes I don't really feel the deficit from not summoning him. I barely even remember his abilities, so sorry if I mess something up. However, maybe to increase his utility, give him a leadership that increases the durability of trolls (including himself?). That could both increase his survivability and make the trolls stronger, which I think would be worth his price.