Deceit and confusion provide favourable environments for the scum to flourish. It keeps the Lightside from getting an accurate read on happenings, while providing fallback for the mafia. After all, if it is now considered acceptable for people to lie, it becomes incredibly difficult to catch the mafia. They can simply admit they lie and invent some other lie, and what are we going to do about it? There's no reason to suspect he's mafia any more than any other liar.

And what do we gain? Possibly saving a role? Roles are greatly overvalued, they don't come near to the value of being able to catch a scum in a lie.