What i mean with the CD key is that the key is matched with the "official servers" so that only that cd key can play on the server (at any one time).
If pirate wants to generate cd keys, it won't work because the server doesn't recognise that cd key.
All the pirate can do is
1. play that game offline
2. play that game online on an unofficial server with other unofficial people - this is fine with me, because the developer should make the "official" server more enticing experience (e.g. number of players, low lag etc etc).
that is how to tackle pirates. But obviously developers will always think they spend too much time on developing the story in the offline game and so don't want to lose it and always think of piracy....