I use to think that with regards to the legality of using emulators the fact the games were that old the makers wouldn't be that concerned as you can no longer buy the games, but with the latest craze of Sega brining out the Sega classics console and Nintendo releasing the Nintendo mini classics with the SNES mini coming out in September it seems it is still big business.
Retro gaming is indeed hot at the moment, but I think emulators have something to do with that too.