I think the general purpose of the skill is to make life in a low head count all mate cabal slightly less rough against battle by partially bypassing spellbane. Shifters don't actually have that inherent problem so while the skill is a bigger help for other classes I don't really see why something would be thrown to shifters that don't have the underlying problem the skill is meant to address.

I haven't seen the skill in action so I could be totally wrong in my assumption that it affects only/primarily spellbane but even with that said it's goal seems to be on assisting offensive casting which is not a primary part of the shifter kit.