What I'm saying is that spending 5 minutes back-tracking through explored areas isn't nearly as effective as simply joining/starting a new game.
If you have 2 hours to play, exploring the whole map won't result in better loot than playing the game without exploring every nook and cranny.
I believe Bashiok said it himself that Diablo III is all about efficiency. Why spend so much time exploring everything if you can simply run through the same area twice?