The examples in this thread could be handled with an extra pair of lexical entries for “on” and “off”, but these aren’t the only prepositions that have this adjective-like behaviour, e.g. as well as “the on position”, we could also have “the up position”, “the down position” etc. Predicatively, there are other examples like “the time is up”, “the game is over”, “the server is down”, etc. I don’t think there’s something special about “on” and “off” (but of course I may have overlooked something). If prepositions regularly behave like this, it makes sense to deal with this using syntactic rules (rather than extra lexical entries), and I think that’s what the ERG currently does.
As for _state, it contrasts with _dir, as two different subtypes of _loc. My vague understanding is that some verbs select for one or the other. In “the light is off”, it’s not being selected for, so it uses the underspecified _loc.
Perhaps @Dan could comment on the above points with more precision than I can.