There's an argument to be had about FOSS vs. proprietary software in general, but I think it's pretty clear when it comes to emulators. There is something wrong with closed-source emulators. Everyone who develops an emulator for a platform is working towards the common goal of preserving the software that runs on that platform so that people can continue to use it well into the future. All emulators build on research about the platform that has come before them, but if they don't release their source code, they don't advance the state of the art themselves. They're only useful for as long as they're actively developed and their closed-source binaries are compatible with modern operating systems, which means they haven't contributed to preservation of the platform.

Also, it's not like proprietary emulators are neutral towards the goal of preserving their platform, they can actually be detrimental to this goal. If Win3mu becomes more popular that QEMU/Wine, users will raise bug reports against Win3mu and certain issues will be fixed only in Win3mu. When the Win3mu dev loses interest or the platforms that Win3mu runs on become obsolete, those fixes will be lost. God forbid people will have to start running emulators on emulators when the best emulator for a certain platform stops being compatible with modern operating systems.