I don't know how well it works in games with only 1 scum player, but with at least two just the fact that there are two players who know they each have a partner changes their behavior enough that the game isn't random. There's also some change in what people say just because each side has a different win condition, although again this is less true with just one scum player.

As just a simple example, when you're playing as the scum it can be really hard (at least for me) to make a good argument that someone I know is a normal villager isn't, which can be enough for another player to deduce my role.