It takes 24 wounds on average to kill a monster with 12 total wounds and a 4++ "ward". That seems like a doubled durability to me.
Paying 100 points to give a 300 point monster doubled durability with a 4++ "ward" save and some cool bonus abilities is beyond a steal. I just don't see how the point system takes into consideration the multiplicative effects some of the rider options for mounts give.
Regardless, The fact both my Flamespyre phoenixs and Dragon Archmage suddenly became good units, or at least relatively equal to their Frostheart/Prince counterparts, is awesome.