On the subject of emulators, yes, some can play legitimate media. Optical-based media anyway. A bit difficult for cart-based consoles like the NES/SNES/Master System/Genesis (Megadrive) obviously.
But assuming you are playing legit copies of, say, Saturn or PS1 discs on an emulator, the issue of legality arises in the BIOS dumps that most emus require. Dolphin is interesting in that regard, as it doesn't seem to require any BIOS rom/s.