Car rental additional driver query

My husband and I will be renting a car in Spain for only 4 days. We will be getting our insurance with the American Express card. Regardless of whether the insurance is issued by the car rental company or someone else, like American Express, it seems that there is no coverage if there is an accident and someone other than the primary driver is driving the car unless that other driver is listed is an additional driver at an additional cost. I do not plan to drive the car, but there is always the possibility that my husband could become ill and not drive for some reason, and I would have to take over. This did occur once on a prior trip but we had no accident. To be honest, I don't know if I would have been covered or not. I'm wondering how other people handle this situation. Just take the chance and hope for the best? Or pay for an additional driver even if not needed? The additional driver cost will be about $60 for 4 days. Thanks for any input/advice.