To further encourage the social aspect of the games that's been a significant part of reaching certain milestones from the very beginning?
I have to agree with Shadow_V-Creator, it was a dumb idea to make the modes version-exclusive and unlocked at the end of the game. They already have more than enough social aspects to the game that actually make sense. The entralink, for example, is a sensible social aspect, as is the age-old tradition of trading Pokemon. Exploring someone else's game, while it's a pain if you don't know someone in real life that plays, is a really cool concept and it naturally fits into social interaction. Trading pokemon implies two people interacting with each other to benefit each other's pokemon collection, and the best part is that you can interact this way with people all around the world, not just someone standing a few inches (in the case of IR connection) away from you. Black City/White Forest are another sensible, if somewhat annoying, way to encourage socialization. Your goal in each place is to increase the amount of people living in your City/Forest, so it makes sense that you have to interact with another game to find residents to move into your City/Forest. It sucks that, again, it's dependent on two people being in the same room at the same time for it to work (unless you have both versions and 2 DS's yourself), but whatever. For people that can take advantage of that function, it can be really fun.
But a game mode is different. It doesn't have anything to do with how you reach out to other players, it is simply a way to increase or decrease the level of difficulty in your own game, against in-game trainers. I think it was silly of GameFreak from the get-go to decide to make this something you had to unlock by beating the game, because Pokemon is not the sort of game that lends itself to restarting. This means that, unless you can get the key you need from an already-beaten game, you can't really enjoy the different modes very much. You can only use them post-story. That in itself is pretty lame. If they're going to give us modes, they should at least let us enjoy them to the fullest. But then they went one step further and made it so that you have to find someone with the opposite game as yours (unless your the type that buys both version) to get both modes on your one game? That makes no sense. I agree that it's something that is in keeping with the style of Pokemon games, but there's no real reason to make your in-game, non-interactive experience (battling in-game trainers) dependent on interaction with another human. That's just silly.
/rant
And if it seems like I'm ranting at you personally, BCVM22, I'm not. xD So, sorry if I came off that way. These are just my opinions, and it's something that I've felt since I first learned about version-exclusive modes. I think they had a great idea when they came out with the modes, but I really think they mucked it up royally.