It's not about using more or less non-ASCII characters. It's about saving the source code files all in the same encoding (UTF-8) and standardizing around this encoding.

They want get rid of an encoding mess.

I don't know how one can be against this. Your code is probably saved in UTF-8 right now even if you don't use non-ASCII characters.

It's probably just that the JDK is a very old code base that was written from different OSes, without much thought on the encoding. Now, they want to standardize while making sure not to break anything.

What's more, the presence of non-ASCII characters in such a huge code base is almost inevitable so yeah, it matters if you want to avoid issues.