Light-churches hope the world will see their good deeds and glorify the Father. Light-churches beam out hopeful light. Churches adopting the light-alone motif may be beautiful and bright and pure but they are usually useless. In setting themselves apart from the world they became irrelevant. And the valley remains dark.
Light alone is too static a motif on which to hang our entire relationship with the world.
It is far more active and invasive. It is a pervasive noticeable preservative. Salt arrests corruption. It stays decay. Salt churches believe the world is rotting and we have got to stop it.