In some sense they've done it already with the Dream World and Event Pokemon. Basically they coded in the abilities and the existence of these Pokemon and locked them behind an event wall so that later on they could release them to the public. That's found out that Arceus existed in the 4th gen before the events were released, cos it was already in the game.
There's a couple of smart reasons for doing it (though whether they're good reasons is up for debate). Money (if they decide that they want to release paid DLC) and retention. The longer they keep you playing the game the better. It kind of sucks if you're a consumer, you have to wait for all these Pokemon to be released and if you don't want to buy the DLC you're crap out of luck. On the other hand, if you're a business it keeps the lights turned on, keeps your staff fed and keeps the public aware of your brand. It's kind of good and kind of bad, the company gets to keep making bigger, better Pokemon games, but you have to put up with the bullshit (alternatively, a big enough public outcry can force a company to change its policies, so if this does turn out to be the case, raise your voice after the game's release and they might change their ways).