Now I realize having casting break combat expertise has good intentions, otherwise you just gave every caster +5 Ac as they dont care about their mellee + to hit. However, this really doesnt make sense when it comes to a monks special attacks. Monks should be able to sit in either power attack or combat expertise and spam off whatever finishers they please without having to reapply CE. I don't think anyone would disagree with me here.
As it stands, it would be difficult for a caster to attain a meaningful enough AC to even warrant getting CE. This may not be the case with druids if they are ever available, but they arent. And when I say "caster" I dont mean a self buffing FVS/cleric mellee type, I mean someone whose primary role is to be actively casting spells. Since I'd assume finshers are coded as spells and thus break CE, just do us all a favor and remove that aspect of CE. Just about every caster would still continue to complety ignore AC and CE and if a few oddballs decided, "hey I want to be an armored/AC caster" who cares? That means an investment into feats,dex and int for everyone besides a wizards, so the extra AC isnt even a complete freebie.
Removing the spellcasting cancelation part of CE would help the classes that actually use/need it, and for those who would potentially "abuse" it, GOOD! It's about time we have the option to make a different kind of caster. Even a pure caster type cleric for example, they were never intended to be as squishy as they are in mellee.