It still combines inches with mm, especially when you have .5 inches involved, and mm and inches that can go in either direction.
This would give a clear idea on both sides of the rim, right away, with no arithmetic. Even better if somebody gives you the dimensions of the arch of multiple cars. i.e GTR may be 125mm, a A80 Supra may be 117mm, or something along those lines.
Yes, you can 'know' that going from a 10in rim to a 10.5in rim with the same offset moves both sides about 6mm, but you still have to 'know' that and do the math. Often it's combined. People are going from 9.5 +27 to 10.5 +15.
You may do the math to know it, but if it was going from (I had to go look it up to be sure) 241mm/2 - 27 - 93.5mm from the center line to (more math) 266/2 - 15 (118mm) from the center line.
Versus 93mm vs 118mm. It's right there. If you know you have a GTT with 100mm guards you can see right away that one is close to flush and the other absolutely won't work. And when someone says "Oh the GTR is 120mm" suddenly you see that the 10.5 +15 is about perfect. (or you go and buy rims with approximately 118mm outward guard space)
I think it's safe to say that given one of the most common questions in all modified cars is "How do offsets work" and "How do I know if wheels will fit on my car" that this would be much simpler...
Of course, nothing will really change and nobody is going to remanufacture wheels and ditch inches and offset based on this conversation :p We'll all go "18x9+30 will line up pretty close to the guards for a R34 GTT (84mm)" but 'pretty close' is still not really defined (it is now!) and if you really care you still have go measure.
Yes it depends on camber and height and dynamic movement, but so do all wheels no matter what you measure it for.