I actually found tenebrium weapons to be overrated. They are handy for the enemies that a vulnerable to it, but in general I regretted spending a lot of points on tenebrium.
So I think the game game is completely playable with you spending points on the the regular weapon slots and then near the end of the game spend a couple of points on tenebrium. The end of the game is also the easiest and so you definitely need these weapons points in the early game.
I think the all debate about tenebrium has mostly been for power players that really wanted to max you the characters and plan ahead. But since the last half of the game is pretty easy you really don't need to.
So play it now and enjoy. I am not a D:OS dev, but I think the design is intended.
