[myusernameisok]

Presumably end users who are compiling from source wouldn't want to enable -Werror though.

If you are an application developer and a compiler X gives warnings that compiler Y doesn't for your application, then ideally you should probably fix the warnings from compiler X. An end user probably doesn't care either way, they just want to compile the program without any issues.

[dmurray]

Realistically, though, an end user doesn't compile anything these days. If you build Chromium from source you're at least half interested in tinkering with it.

[myusernameisok]

Well certain Linux distros (Gentoo comes to mind) are designed to support their users compiling applications from source. Some users just want to be able to compile using -march=native, and aren't interested in fixing compiler warnings that they didn't introduce. I personally would hate it if I wanted to compile an application from source but -Werror stopped complication through no fault of my own.