With the exception of Pong, most arcade games were designed to be primarily a "player against computer" challenge. The goal was to make money, and if that relied on having more than one player available, it would cut into revenues considerably. That's not to say that VS and co-op game modes didn't rake it in as well, but when playing with/against strangers with these types of games, you may as well be playing with/against the computer. I have fond memories of playing Space Duel and other co-op games with a buddy of mine back in the day, but I have no interest in exposing a well-working machine to the nasty possibilities that come with a web connection, just to play with an anonymous individual.
If you have a long-distance friend with similar interests, or are heavily into SFII and the like, I can see the attraction to using something like this. But given that this is a niche hobby, and that this type of application serves a much smaller niche within it, the user base probably isn't going to be very substantial.