Bill, I agree, the trades have lots of little goofinesses to deal with, and that's my point. A framer knows how to space studs to fit the standard size products as they exist now. He knows that 3/4 ply is actually a little less, and that a 2x6 is actually not. It's a system that has warts, but works. Throw in a whole new level of craziness by changing the units of measure, and see what happens. Shoot, I make my guys use story sticks as much as possible just to avoid some of the measuring screwups with a measuring system they "understand" already.
George illustrated the problem well when he said "40 cm". This actually has nothing to do with the metric system, it's actually about the industry standards. As the engineers, manufactures, and trades develop and incorporate product into the standard, it doesn't just happen. It's a process that works for each level. If one of those levels finds the product decreases the profit margin, it won't get used. I use Schluter products often, I can buy Kerdi board in several sheet sizes. I lay out tile in metric because it's easier math usually, but I buy the larger 4x8 sheets because they fit the room better and have less waste. Schluter makes the product in Canada, but the sheets raw sizes match building standards (4x8). If they didn't, I wouldn't use it.