Combine the issue of tolerances with the fact that even on American cars, you aren't guaranteed that *every* nut / bolt will be of a particular (i.e. US or metric) type, it starts to get to be a pain... Back in the old days, I knew that I needed a set of metric wrenches for a VW and US wrenches for my Pontiac... Never did I need to consider using one type of wrenches for the other type of vehicle... These days, my Jeep seems to have a bit of every type of nut and bolt... I've pretty much just come to the conclusion that I need to have both sets available to me and try each type of wrench out to see which gives me the most snug fit...
Hell, I don't care if a vehicle uses the metric or the US systems, but I sure would like it if they could at least be consistent...