I don't think of DLC's purpose as saving a shopper the trip to the retailer so much as saving money on both sides of the transaction (the developer and the gamer I mean, not the retailer) for development/print reproduction/whatever costs and whatnot. I imagine a shopper and retailer still get this benefit when buying a DLC code from a retailer.
But given how many DLC games there are out there, I can't imagine a retail store ever really having a fraction of it, I would just stick to the hard copies myself, especially since it's already hard enough to get some of the rarer games still in print from them sometimes.