^That is not true.
You can, for example, add tenebrium to weapon for high damage output (more then you can get from adding elemental essence or poison flask), then add elemental essence or poison flask on top of that, to convert all tenebrium damage back to elemental/poison one.
This is useful trick in cases when you heavily invested in normal weapon skills, and would rather exploit tenebrium to improve damage of other elements and keep weapon using normal weapon skills.
