In my opinion, death notes are more of an entertainment factor of Mafia than a gameplay/deception aspect of the game. I would love to see town killing roles have this feature.
You may argue that it gives town a way to communicate, but actually it doesn't. Veteran can't give information. Vigilante wouldn't shoot an invulner target and then kill someone at random to reveal that his first target was invulner. I might agree that jailor can abuse this ability and that bodyguard doesn't even need it.
Remember that this is merely an entertaining aspect and not a balance aspect.