So I'm just checking out the forums and I notice this thread. "No," I think, "that's just too stupid to be true." So I start up the game and find myself a +1 Bow item so I can check out how my archer's damage changes with her Tenebrium bow. No change whatsoever.
So then I think, "holy crap, it really is true. Wow, is that ever insanely stupid." (But with more curse words involved.)
I'm not sure what the fix is, but the point should be: being skilled with bows, or one-handers, or whatever, shouldn't just go away because it happens to be a Tenebrium weapon.