OK, try and quarter both those numbers. Quickly.
If I just need an approximation then it's pretty quick to get 400mm. And an exact figure doesn't take all that much longer. 5'6"1/2 I have absolutely no idea where to start. Not only am I dealing with fractions, but I'm also having to deal with different bases.
I guess it comes down to what you're used to, and you would develop various mental shortcuts if you worked with these lengths everyday. But switch to weight and you now you need a whole new set. With everything in decimal those hacks need to be developed just once and they get used everywhere and hence become much more efficient.
It might not seem a bit deal but quick mental approximations are essential for spotting mistakes.