I'll tell you something I do as a woodworker a lot that metric isn't great for: divide by powers of two, three or four. I've got some boards milled up 3/4" thick. I'm going to join them with a bridle joint, that means cutting the middle third out of one and the outer two thirds from the other. So each of the remaining "tongues" are each 1/4" thick. 3/4" is approximately 20mm. That's a nice metric number, a multiple of 10. Let's cut that same bridle joint in 20mm stock. What's a third of 20mm? Can you come point to 6.6666mm on my metric tape measure here?

Don't pretend base ten doesn't do stupid things too.