? The current vector beta definition in LESS[1] is
   @content-font-family: "Helvetica Neue", "Helvetica", "Nimbus Sans L", "Arial", "Liberation Sans", sans-serif;

Most popular Linux variants specify an equivalent FOSS font for "Helvetica" that ships with the OS for exactly this scenario, ensuring that users get a decent approximation of the proprietary font's appearance by some FOSS font. A few brave users customize the matching behavior because they prefer something else, or they read some how-to article. If we put the free names first, we just frustrate those efforts, and the experience of 90% of our users doesn't change.

faidon:

it's basically undefined behavior

A font stack is inherently undefined behavior :)
Yes we get somewhat unspecified behavior for a small subset of our users, but on balance it's better and more freedom-y to let them evolve a better FOSS version of the notion of "Helvetica" than nailing them to 2012's fallback "Nimbus Sans L".

I don't know any free systems that specify "Helvetica Neue" equivalents as well as "Helvetica", I don't know what Android does.  (Can people spend less time hypothesizing and more time reporting their /etc/fonts rules and experiences with the Vector typography refresh Beta experiment[2]?)

Quim:

And if we want to specify any fonts in our works, they should be free.

Uh, why? Mac users actually have Helvetica Neue, the nicest-looking font, Windows users have Georgia. The presence of these names in our CSS does nothing to hinder the cause of free fonts. Removing them would be detrimental for most of our users.

The problem with giving up and sticking to

serif, sans-serif, monospace

is designers can't advance the appearance of MediaWiki. We engineers engage in all this scholastic argumentation over font names, meanwhile the Vector typography refresh[2] is a big f***ing improvement to the experience of a billion MW users! I wish we had a video of Vibha Bamba's passionate defense of design from the tech days meeting.