It's very intersting, why don't everybody just use knots and nm? Wouldn't that be easier for the pilots and for ATC?
This would of course be ideal & avoid possible mistakes. In fact I believe it is usually the case with commercial aviation, just as English is the international language used for radio communication.
A lot of things would be easier if we all used the same measurement systems & spoke the same language but we don't. From personal experience, incorrect conversion from one to the other is responsible for a lot of accidents.