Technically you don't BUY software, you only License it so they can remove their license (and therefore your right to play/use it). So in the case of a disc, you'll at least have that but in the download (and if online requirement) they can just block your ability to play it (no refunds either). Surely they can do this for disc too if the system was updated to a firmware that blocks said disc but still.
Also this is relevant: UK Digital Download prices
$47 for Angry birds?? WTF