I'd also really like to see the various polymorph/shapechange spell shapes actually get to use items like the druid/shifter shapes do. None of the shapes have weapons and you can't cast tensers and then shapechange, or vice versa, just to get free crit immune and keep high caster levels.
You can actually cast Tensers after polymorph, although with no weapons it wouldn't do much anyways. If, however, all gear did shift including weapons, then it could be a problem. I don't really see any need for this to be nerfed except for the benefit of NPCs.
So far, anytime I've fought Radagast, he casts Shapechange and will continue to fight without spells. This basically strips him of all his gear so that he is: easy to hit, doesn't hit back (and if he does its really weak), and I'm not sure if it effects his drops (essentially dying without items).
An easier fix, in my opinion, is just to remove the spell from Radagast.