Well lets face it, it is lame. And if you don't like it, the simple solution is to download a cheat engine table and just convert your 1h/2h/bow/xbow level in to Tenebrium once it feels appropriate to do so.
Actually the simple solution is to abuse the ability by using it exclusively. This mechanic only punishes the people who didn't know better and invested in the normal weapon abilities.
It's a better weapon ability (since it doesn't discriminate on weapon type, and you get a free bonus from quest) for better weapons (see my post to Casta about the numbers).
