And 5 really isn't that much for workshop decks, or control decks that want to drain into him. I don't know where else either would see play anyway.
RM Masticore in control decks seems like an intriguing idea. A 5/5 First Striker with built-in Bolt for 5 Mana is a bargain. Still, I don't see the need. It is a BIG blocker, and smashes for good, but I don't see the matchup where this is necessary - yet. U/G and other classical aggro strategies are easily stopped by FTK/ Exalted Angel, so this is not the place where you need it. Also, the discard is a serious barrier that IMO prevents the RM Masticore from getting "just thrown in".
Where I can see a necessity for an 8-damage blocker in a control deck shaping up is against Slaver executing the 7-men plan. FTK doesn't look so hot there, and having a hopefully permanent blocker which can remove most of the threats is intriguing and probably worth the card-disadvantage. Still, it not a sure inclusion. The vulnerability is high, and 5 mana is slow compared to Workshop acceleration.
Against Tog, it might screw their game plan, but so do the cheaper and less vulnerable Angels by raising the cards needed to kill by two with every hit.
Anything else (even Wurms and Roars) is manageable without the additional risks of RMM by sticking to ones guns. If it has a place in a Control deck remains to be seen, for now, I am pessimistic.
Dozer