So I asked Mearls on twitter something that had been bugging me since basic released, if Rogues can only sneak attack with finesse weapons why give them longsword proficiency. He responded with, "So they have something to do when not sneak attacking." This just seems a little silly to me, as a Rogue would be using a Rapier not a Longsword anyways if they wanted the d8 damage dice, so it strikes me as odd, especially with the response earlier to allow any d6 or lower weapon to be able to sneak attack.