It's an increase of what the whole series has been about: making people trade with, or, what they expect a good portion of people to do, buy multiple versions of the same game to get everything. Just look at the last 'generation' of the games. You would have to buy six different versions to get all the previous monsters (Ruby, Saphire, Fire Red, Leaf Green, Colloseum, and XD). Emerald wasn't really needed, but in order to get some of the bonuses, which I can't remember right now, you would have to buy that one too.