MS' Phil Harrison has confirmed XB1 discs are worthless
Once you have installed and activated the game, the disc is no longer needed. If you take it to a friends place it will be installed on his HDD and then he can buy it with one button click later if he wants. Any potential business with license resales is still TBD but best case scenario it will not be kind to GME's margins, and could be subject to change by MS at any time. Worst case, there is zero used market for XB1.
PS4's strategy is still TBA, but I expect several pubs there will ship one time only activation codes with the disc, so those discs will also be worthless.
All XB1 games will be available digitally at midnight on day of release.
So it isn't just the likelihood of MS and Sony taking more digital sales of new games, the implications for the used market look dire.