The absolute best thing about NXE is being able to delete the arcade trial games from your achievement list that you don't have any points from, nor likely to ever get any from.
Aha! Good to know, I'll be able to clean up my list a bit
Edit: How?
It's likely just for copy protection - Stops you renting and installing games then playing them indefinitely.
What I meant was that Microsoft could only allow each game to be copied to one console by linking the disc's ID to your account. If you then tried to copy it to a second console, it would ask you to remove it from the other console first.
To be fair, if I was doing it then I'd allow three consoles per game, for families with multiple Xboxes etc.