I have a question about the serial key actually. Would the game actually allow two players with identical serials play a game together? If not, would there actually be a problem if that were actually not allowed to happen?
1) Yes, two people with identical keys can currently play together. I know, as I did this with the friend who introduced me, prior to buying my own copy.
2) Actually, yes. If you activate with one key, there's no way to "revoke" that key and/or enter a new, different one (ditto expansions).
See, that's something that doesn't really set well with me. Typically even games very lax on DRM won't allow two people with identical keys to play together. Me, I just won't do it because I'd rather buy that person a copy of the game to play with that person, or even to show that person the ropes. But, the fact that it's there makes it too easy to spread one key around and have full and complete access to the entire game for free, and you can just play online with anybody you want. It's almost too free. Seems preferable to make it such that the game checks that two players' serial keys aren't identical in the lobby... but like, in practice, it still would probably have no noticeable impact on piracy rates.
It really just doesn't set well with me because of how crazy easy it is to just have a free copy of the game. I don't even see how it would be obtrusive. It'd only ever happen if you played a game with someone who had an identical key. You could install the game to as many computers as you want without any issue.
And yeah, the steam version can be launched without launching steam or the steam overlay. You just run the AIWar.exe and you get the game just like that. In fact, I think the auto updater launches the game in that way when it automatically relaunches after installing an update.
...all of that said though, I wouldn't support even this unless it would actually be worth the effort to prevent players from essentially giving away the game infinitely.