NWN2: Combat Expertise

[Black Swan Gauntlet]

One of the nice things about NWN2 is how the various combat modes can be toggled, rather than deactivating every time you disengaged or ended a fight. It does get me thinking, though - Expertise confers +3 AC and a -3 penalty to attacks, but that's irrelevant if you play a primary spellcaster character. In essence, you get a better version of Armor Skin at a low level (and even better still if you spring for Improved Expertise.) So, a few questions:

1. If playing a sorcerer or wizard (or perhaps a magic-oriented divine caster), is there any reason to ever turn this feat off?
2. As a side note: concerning warlocks, Eldritch Blast requires an attack roll, so I wouldn't pick Expertise quickly. But if I use Eldritch Cone or Eldritch Doom, is this still the case?

[Gabrielles blades]

using combat expertise instead of combat casting will make them get attacks of opportunity whenever you cast a spell. So even if the extra AC takes you beyond the normal ability of the creature to hit, they still have a chance on every attack of opportunity to critically hit you.
i am not sure if it affects the ranged touch roll; i think it uses just the BAB.
Note - it seems as though damage resistance may be more important than AC since a high resistance is always there and will counter the dmg from attacks a lot more effectively than higher AC will simply because of the fact that a critical hit always hits regardless of if its attack roll was 65 and your armor is 77.