Aikouka
Lifer
The only thing that I can think they were thinking was to start a game in nightmare with some buddies and then join as a 1st level and skate on the loot until he had uber stuff. Then jump ahead on normal and wrack up buku XP by fighting stuff that is WAY over their head.
Because, um.... 1st level against Nightmare creatures?
I don't think that's the case at all. In most games (i.e. not Diablo or Borderlands), to play a higher difficultly level, you typically just start a new game at that difficulty level. In some games, you can swap difficulty mid-game. For example, Torchlight II lets you set the difficulty when you create a new character.
Diablo and Borderlands differ in that the extra playthroughs offer the higher difficulties.