The inherant flaw with DD is your purchase has no equity.
Games are finite. Like food or water. You only play them so much, and then you move on.
Now, on consoles(besides the PSPGo) you can take your old pile of shit, and flip it. Sell it to the person who wants it.
Then you can use that money how you want. Potentially to buy another game, you haven't played yet.
People like doing this. It's an extra option. Something that not everyone will take advantage of, but some will. It's what makes console gaming cheaper than PC gaming. It's an entire industry.
Your inevitable envisioned future of DD only gaming isn't inevitable because it furthers gaming, or because it is better than the present in any way whatsoever.
It is inevitable, because it makes the gaming companies MORE MONEY, and screwing over gamers never hurts the industry.
If you think FFXIII is gimped because of the 360, just wait until Sony starts putting size limits on games they'll approve, because of their DD strategy.