I am not aware of this skill's progression in particular, but I haven't seen any skills that progress based on level. I have seen several HP related skills progress based on max-HP, and a few damage over time or cooldown add-on type skills that progress based on Attack Speed (since Attack Speed is considered less useful on those skills). A couple such skills I'm certain about currently are the Monk's Fire Elemental Ally's Additional proc and the Demon Hunter's Grenadier Passive Skill's proc on death. I wouldn't think that skill would be the lone skill to progress based on level.
However, though I don't use the skill personally, I don't find Entangling Shot to be overly weak. In my opinion, all of the Hatred Regenerative skills are going to be weak, with possible exception to Hungering Arrow... That can usually have a better effective range and become useful regardless. However, in general, you are going to want your skill to help pad the time between power spurts, and the utility involved could arguably do this better than other generation skills. Furthermore, I haven't used the skill much, but multiple units being hit I believe means more life per hit to you, though I hear AOE drops the significance of this factor, and I wouldn't be sure if it applied to this skill. However, you are going to need a lot of power to back up this much weaker skill.