I travel with two debit cards (business and personal) and three credit cards (one business, two personal.) I've had credit cards cancelled mid-trip or not had them work to make a purchase (service stations in the U.S.) and had to resort to a debit card. I've also had debit cards not work to withdraw local cash (Namibia, French Polynesia.)

In places where theft risk is high I will split up my cards and leave one debit and credit in my hotel safe in the event I lose the others.

IMO, what is really crazy is travelling without enough room on your cards to manage an emergency or an unforeseen last minute change in plans. I have high limit cards that can get me back home from anywhere in any fare class at a moment's notice.