I ride nearly everyday and my rationale is:
Better to be safe than sorry. If there is any chance for considerable ice on my ride for that day, i will be sporting at LEAST the front studded tire. If there are definite icey conditions around than i will have both studded tires on. There is really no reason not to have the carbide tipped Nokians on, even if much of the ride is dry. The only downside is it slows you down a little, but not nearly as much as a broken pelvis or hip will.