I see good sorcs with CtA in a staff with +ES and other adds. I recognize +3 to certain skills is good for buffing but isn't the +2 to all skills offered by a spirit better?

This will give them the opportunity to use something Like Energy shield as a Cold/fire sorc or Shiver armor as a fire/light sorc for free which saves them 4 skill points.

It's a better trade than the 6% life/mana gain from 2 more levels of BO

spirit doesn't give you energy shield
staffs give you energy shield