Plain material is suppose to draw without lighting. But it has some issue with ForwardRenderer.

It's mostly used for Ui or Debug, which seems to work fine (since there is no lighting in these rendering).
But if it's a geometry component, the ro is added to the light loop, even if it has plain material.

On simple solution is make plain material only affect ambient/zprepass, but it's not used in ui/debug rendering.
Another solution is to refactor the way ForwardRenderer choose how to draw things.
Or simply wait for the "new renderer" that can fix this (hopefully?)