Despite the detail that I don't understand why users should have to
bother about a possible semantic difference between "Disabled" and
"Cleared" in the two path settings.
The PD Documents Directory is a built-in plugin that has extra functionality which can be disabled when it's value is "DISABLED". This could probably better be a check button, but I didn't want to take more vertical space.
The Externals Directory is similarly used by a built-in plugin, deken. Clearing this to an empty string says "let deken use it's default behavior".
This current set up is an interim location tight now. A longer term plan is a more integrated Preferences panel with tabs which could have better contextual grouping.