Just did my first playthrough, tbh didn't pay so much attention to the actual weapon damage since I was convinced that both skills would determine damage. I'm quite surpised this is not the case.
However all this talk about tenebrium being so called "superior" weapons is not really true. You find a alot of non-tenebrium high dmg weapons in late game, even legendary ones. I was specced +5 one hand and +5 tenebrium and I even ended up using a non-tenebrium weapon in the last battle because it was superior damage. Didn't craft any weapons at all.
I agree though that tenebrium skill should be reworked, because wasted points is not good, however it should not buff man at arms users. I found my man-at-arms user to be extremely hard hitting (flurry, battering ram, whirlwind) and had the best synergy with talents. I mean weather the storm is completely insane, 50% resist all elemental damage permanent and picture of health 25% extra vitality, bully 50% dmg vs. some disabled.... None of this insane talents were available for magic users.
However magic users is not to be called bad either, meteor storm is the most hard hitting spell in the game, even on single targets, the witch finger or whatever its called is not too bad either.