Roleclaim to the leader and follow him will eventually lead to players learning how to became one of the people who the leader trusts with all the roleclaims. But it won't work so well in setups where the list of roles in known, because then everybody would know that SOMEBODY is a mafia member who appears innocent, etc. Now, if the roles in the setup are completely unknown, it is in the scum's best interest to became trusted by the leader. They may have a good role (appears innocent, etc), allowing them to do this, or they may be able to come up with a good fakeclaim etc. Or the scum could try and subtly prevent the village from getting a leader in the first place.
tl;dr version: Scum will learn to either subtly prevent a Town Leader from emerging, or they'll learn how to gain access to the info.