Someone is making the argument that by capping out and ending a game early, the minutes spent "killing all" could instead be spent earning 'fresh' XP in a new game and, in the long run, that earns more XP and credits. Similar to what Necrophore says below - higher 'XP per hour'.
I think that is bullshit for a number of reasons, but wondering what the data says.
According to WN8 creators, it doesn't. http://wiki.wnefficiency.net/pages/WN8:_FAQ