I don't know if Tenebrium was originally handled this way during concept or alpha. For example, there is one unique dagger (Snakebite) in the game that comes Tenebrium damage. One of the modifiers on the dagger is +1 handed; there is no +1 Tenebrium on the dagger
If the normal weapon abilities weren't supposed to modify Tenebrium weapons, then this unique weapon was made as a slap in the face for rogues.
Also, why bother separating bows and crossbows in the regular damage trees when there is no separation for Tenebrium bows and crossbows?
@Vedros
If you are going to respec, I suggest hunting for skillbooks

@LordOfBOnes
Also, how about the talents Bigger and Better as well as All Skipped Up? Those 2 talents seem to be made to encourage people to respec.
Funny enough there was some talent balancing that nerfed the rogues. If you look in the code there is a talent for rogues that reduced basic attack AP to 1.