Runaway inflation is a dubious reason at best. I suspect the official line, if you could ever find someone that knows it, is that the DCC can benefit the consumer by stating the cost in their native currency so that they know how much the purchase "will really cost" without having to do the conversion themselves. It's true that there are certain travelers that just can't latch onto the concept of currency conversion and can never figure it out. Of course, what is left unstated in this equation is how much of a value this is versus the conversion rate applied to DCC.