Online co-op for classic games on newer consoles is very cool. 
Sure you can do it through computer emulation, but losers like me don't play games on computers, and its cool to hook up with people through Gamertags and you can private chat while you play. 
It's just ease of use too on the 360...it is very easy to see which one of your friends are online and what they are playing. Then you can just send them a request to play a game then pow, you're in.
BTW, if anyone has sigs turned off then you can't see my gamertag banner which shows my gamerscore, games recently played, etc.