At first I didn't think much of this, since if a wizard is in melee, the extra ac from expertise or improved expertise would be great. But then I thought "What about those PvP scenarios?" Depending on the situation, if a guy in full plate fought a wizard in a duel for example, the wizard could just use expertise/improved expertise and have more AC than the fighter, who needs a high roll to hit the wizard, and now all the wizard does is cast spells because he doesn't use an attack bonus. So I can see the understanding of expertise being canceled while spell casting.
We did Ogre quest today with a wizard. He got his AC to 49? Or something stupid high like that. Full buffed. Now stack on expertise on top of that for the LOLs. It gets pretty ridiculous. Haha. I don't remember what his AC was exactly, but I DO remember that I had a +30 AB, and thinking "Wow, I still wouldn't be able to hit him without rolling high on the dice."