Some skills doesn't incentive you for level up them

I saw an interview where the devs are trying to incentive you to level up all the skills in the game, but this isn't the case in all the skills.

The clear example of this is the "Cull the weak" skill. The skill only scales damage per level, and isn't worth it. For example, cull the weak should have more attack speed per gem level or additional chance for obtaining a frenzy charge per gem level.

Some skills should scale other parameters rather than damage. I don't know if this is for the quick add of that skill into the game.

One of the biggest issues I notice with leveling up skills is the mana cost increase. This is especially painful for the classes that need dex and str since neither of these stats gives more mana.