was it for convenience reasons (didn't want to deal with 2 credit cards)? because from a purely rational point of view, using debt/cash when a place doesn't take amex is always worse than using a no yearly fee visa with some amount of cashback.
I almost never ran into a merchant that didn't accept Amex, so I didn't have any particular reason to go to the trouble of getting another card. In fact, I more frequently ran into places that didn't take credit cards at all.
I didn't, for several years, until a Visa card came out with better terms (1.5% cash back) than the Amex variant I was using (1%).
Also, some companies provide an Amex Corporate card for travel expenses, and it's less convenient to charge things to any other card.