The difference between a used digital copy and a used physical copy is that the physical copy gets damaged with time, the digital copy remains the same, therefore there would be a true competition to the manufacturer himself. For someone like steam which has so many deals I can see it being unfair.
Then again the right to own stuff is also important. If you own it you can sell it makes sense, however don't expect this on PSN and XBL at all.
Edit : Also don't expect the free redownloadable option you get with the current download method. You buy a game you can download as many times as you want, that can't be done with a physical copy. And when you sell a used copy you would have to sell it through your HDD, not the publisher's/platform holder's servers since they have nothing to do with the transaction anyway.
