There is one quest that requires Tenebrium weapon, regarding some blood stone (I would not spoil more).
As of now, it is best to put points into the tenembarium skill. The other skills are a waste. This is a major design flaw.
My opinion is quite the opposite.
The thing is that this is not MMO. End game power is irrelevant. What is important is player power through whole the game on average.
For first 50% of game, regular weapon skills are always beneficial (at least first 3 points, if not more), and gimping yourself for some end game powerplay is just suboptimal way of playing the game.
Then, when you finally get Tenebrium, my opinion is that player is already invested enough in regular weapons, that it is a waste to invest points in Tenebrium.
Also, while you can convert regular weapon to tenebrium skill weapon using tenebrium bars, which adds more bonus damage then elemental essence, there is an additional trick here. If you use elemental essence on such tenebrium converted weapon, you'll convert tenebrium damage to elemental damage, thus getting more elemental damage then if elemental essence was used as only component in crafting.