The whole point of the American unit system is that they are optimized for specific use, but definitely not for conversion.
ISO paper sizes are a great counterexample, actually, because they took an application, rather than a conversion-centric approach. I.e when you print a signature you do so on a larger piece of paper, and then you fold, crop, and bind. Each fold cuts the paper size in half.
So ISO paper sizes are much more like American units than American paper sizes are.
US paper sizes assume that you have specific design sizes to your printing technology (picas) and then give you a grid of an even number of those units.
Graphic designers / typographers I know tend to prefer the US paper sizes.
Regular people who want to make photocopies of enlarged/reduced pages tend to prefer ISO sizes.
Anyway, the main advantage of metric seems to be its universality between different markets, rather than the somewhat silly idea of ideal relationships such as water in one arbitrary form having a certain weight and volume replacing a system that was derived from a hundreds of years process resembling a genetic algorithm.