Eh. Flails are my favorite medieval weapon and all, but my god, can you imagine programming those in the game without them being the most buggy things ever? It's so hard to include flails in a game with physics like we have today. Too much effort for too little pay off. Spend time on better things, I say.
One look at the way things have worked under Havok physics and I can only imagine flails as painful comic relief. Hit giant for 10 points, flail head Havoks back into you for 25 then begins a frantic match of "ping-pong to the death!"
Well problem thats you see only havoked flails, but there is another way to make them: Animations
So such flails can be done for creatures at last if not for NPC and Player.