I think Wiss is onto something here. I still think it would be viable to take as often straying too close to a unit of handgunners or crossbowmen, heck even some lucky archers can cause your monster to end up a pincushion and it's so frustrating to know that all they need is a couple of lucky 6s and the VPs start leaking out. They are not that hard to bring down with small missile fire.
Warmachines should kill them, so that is ok. Arrows should too in large quantities, I don't deny that, but I think a little protection could make sense if one wanted to take it. I almost always take extra armour over extra guys as it's amazing how often those 5s or 6s get rolled in a pinch.
I like the idea of a few more monsters daring to fly onto the table, they are just too delicate right now though, those with saves already ignored.
Perhaps as extra incentive, the rider could get +1 from it too, like they do with barding on a normal steed...