I never understood why Combat Expertise was disabled when you cast a spell.
It's a combat stance in D&D that is a choice before each turn, and there is no mention of being de-activated when casting spells.
For example, Power Attack or Precision are not disabled by casting a spell...
I would say ok for Defensive Fighting (still should not been disabled by casting), but Combat Expertise should be available to Clerics/Paladins/Wizards too, but because they are spell casting classes they can't benefit from it?
Thanks for an answer on the reason of the deviation on the official rules of D&D on this one.