You wouldn't if you had both cards. However, before CSP became primary, AmEx's car rental protection was usually a better buy and something I did overseas. Now I just use CSP, but it still exposes me to DCC in non-USD denominated locations. Admittedly, the only location where I've rented recently where this is the case has been Australia with Hertz, and fellow FT member and I klashn saw no attempt at DCC.