The difference between placing a pre-order and purchasing a brand new copy is one thing, you cannot get pre-order copies when a game has been released. When games are released, reviews and playthroughs are released that help people make an educated guess on whether the full version of the game is worth buying.
I'm sure people can see where I'm going with this, the fact they used such a major bargaining chip this way, doesn't speak well on their confidence of the game.
I feel compelled to say that, in the past, I've seen that any game with exclusive content that we sell (I work at GameStop), especially something like dated codes for DLC, is usually packed into the new game's box, and not a code printed on the receipt at the time of purchase. Batman's Catwoman DLC was like that - a pack in. Or let's take games that offer Beta codes like Medal of Honor had for Battlefield 3 - usually a pack in.
The chances are high, and I'd wager that DmC's Vergil's Downfall DLC is one such thing, packed into the game's first couple of shipments, and ready for use when the DLC is available. They say "pre-order it" because that ensures that you get one of those first shipments, instead of a later one without said DLC. Either way, it still gives incentive to people who are going to support the companies buy purchasing new a copy. I hate DLC like this as much as the next guy, but this is the way of the industry right now :/ At least there is a way to still get it, and to even get it for free.
As for the DLC easily being fit into the main content, I doubt it, because they've been polishing the game for a while now, and in development, adding in extra content during the polishing phase kicks the whole game back by a few months, and pushing a game past its projected release date has a lot of monetary consequences. Try painting a portrait, and then having someone else want to be in it while you're pretty much done and simply adding in bits of color and shading - it's sorta difficult to do without unforeseen side-effects.
