I think it's just about shrinking loot tables. They want rogues to use fast dagger off hand in all specs. With the change to Main Gauche they are cementing this in as the rest of combat is built around the fast off hand.
Makes it easier on you in the long run as some fights benefit from being combat (when it's not broken) and some from being assassin and you can use the same off hand. Less waiting on loot to drop.
An...eeerrr...one less bag slot used lol.
But yeah doesn't look as badass as two big swords. Guess you got to roll warrior now for that