1. the disk-based version will continue use the physical presence of an official disk to make sure the company got paid for the game (meaning you have to have the disk in your system to play, even if the game is installed on your hard drive).
2. the digital version will be linked to account ownership, meaning everyone who plays it on a different account will have to buy a unique copy (since it's the only way to make sure people using torrents or file hosts don't just share the game with ten thousand of their closest friends without the company who made it making a red cent from it).
So to everyone saying that they don't want to have to pay separately for every account (ie, every person) to play a game: you're already doing that for digital games. The only difference is Microsoft wants (or, I suppose, wanted) to extend that system to all games, regardless of the physical presence of a disk. They don't want to continue with two different methods depending on whether you installed the game from a disk or through digital download. (And maybe, just maybe, we could have gotten to the point where we could play on a console without having to have the disk in, meaning no more disk-swapping.)