[azernik]

There is a reason technical fields should mandate uniformity and compatibility, and that isn't going to happen with customary units (so called because they're not even compatible with pre-metric British units).

Sure, you can fly your plane with bolts measured in inches and fuel measured in pounds, but good link getting replacement parts or accurate refueling next time you're in a stopover in Japan or Italy or Egypt. And have a good time trying to sell machine parts into an international market, or trying to buy from that same market.