I know the Pidgin project used to build their windows version on Linux using GCC and setting the make target.

You get a constantly up to date compiler (because of the system package management) and you don't need a MS license for your buildbot. It's a win win.

If it's a cross-platform projects then no wonder. Also we know quite a lot of cases when developers are working on Windows and using VS, but compile for Linux and use GCC for sure.