Why does there have to be any extra content at all? Why can't you just buy the complete game, whether you decide to buy it at launch, or a few days later? Or if you decide to buy it at retailer Y, instead retailer X?
I get Pre-order Bonuses in form of Artbooks, soundtracks, statues, etc. But game content shouldn't be kept from you, simply because you refuse to buy a game blindly, without knowing the actual quality of the end product.
Pre-order game content, just like disc locked DLC, needs to go away.