I downloaded one, but never bothered installing it, since the game ROMs are a pain to find and take years to download.
Actually, games only pass into public domain if the copyright holder allows them to, or said copyright holder is dead or out-of-business. (I'm assuming copyright laws apply to games, since they are written, after all.)
Nevertheless, I personally see no harm in acquiring games that simply aren't being published anymore. Say, anything from Super NES and before. After all, any actual games you find are (more than likely) secondhand anyway; the developer has already made all they money they're going to make from it.