I know I am necroing this thread, but I have valuable insight to add to this, specifically in regards to Monks.
Under normal circumstances, TWF can be used with unarmed strikes. Monks, however, are an exception do to the following statement in the writeup of the monk's unarmed strikes: There is no such thing as an off-hand attack for a monk striking unarmed.
So a monk benefits from twf only when using a monk weapon in the off-hand.
I'm currently playing a paladin with the enlightened Paladin Archetype and ran into this complication.