The original legacy roles were intended as a short term workarounds, soon we discovered we actually need to use them for setting up of defaults and resetting of roles.

Problems:

• usage of legacy capabilities
• unintended modification of admin role - it should not be possible to override admin role, it should be used only in system context
• negative guest capability - once assigned there is no way to undo it

Spec at: http://docs.moodle.org/en/Development:Role_archetypes