The big deal is that weapons with tenebrium damage on them only benefit from +damage from the Tenebrium ability, not the other weapon abilities like Bow, Single Handed, Two Handed, etc. So if you invested heavily into the latter, those ability points you've spent are essentially wasted once you reach the point in the game where you get access to tenebrium weapons.
But the real problem is that unless you've played through the game before or have read guides, there is no way you could know this beforehand and plan accordingly. So basically the game more or less tricks you into building your character sub-optimally unless u meta-game, which is just poor game design. Maybe it wouldn't be such a big deal if respeccing wasn't such a pain in the butt, but as the game currently stands people are right to complain about it.
