> What does changing the type of the "author" field make clearer exactly? 

It "Author" not "Authors", that's the difference. Like we have
"Classifiers" and not "Classifier".

Plus, I am not breaking any compatibility here.

A multi-value field in the metadata in Distutils just means that you
might find several lines starting with 

{{{
Author:
}}}

And that Distutils will be able to recognized both forms, exactly like
it does with Metadata v1.0 and metadata v1.1.

> I'm bothered that many people currently seem to think that it's ok 
> to compatibility for whatever non-critical reason. This is not 
> the way we will make Python a reliable platform to use and develop for.

I don't know who you are reffering to, but if you look carefully at the
work I've been doing in Distutils, I have never and I will never do any
gratuitously change whatsoever. 

I am bothered that more and more are people constantly jumping on my
back everytime I try to make Distutils evolves a bit or everytime I
discuss some feature proposals with the community.

Any change to the metadata fields will require a PEP 341 change, so
everyone will be free to do a +1 or a -1.

But saying that making the metadata evolve "is not the way we will make
Python a reliable platform to use and develop for" is completely
wrong imho.