There are easy ways to convert between the two systems in some cases, though.

Km and miles:

Fibonacci sequence: any 2 subsequent numbers are a close approximation, with miles=lower number, km= upper number

Inches and cm:

1 inch = 2.54 cm (exact) So, 4 inches = (about) 10 cm or 100 mm

Kg and lb:

2.20462 lb = (approximately) 1 kg

Double number of kg, move decimal left, add the numbers, and you have pounds.

There are other easy (mental math) conversions between the two systems.

Note: I do not personally prefer either system in general— just have to deal with both regularly for work, and have noticed advantages to both.