I think that is close to the equivalent bonus for disadvantage, as calculated by someone on these boards a long time ago.
Using that ability could therefore grant disadvantage instead of -5, but that would reveal the main limitation of the (dis)advantage mechanic: that they don't stack. Therefore using this feat would be the default everytime you already have disadvantage for another reason, resulting in getting the bonus for no additional cost.
The rogue in the current playtest packet offers a solution for this. You can accept disadvantage on an attack in order to get sneak attack damage--but only if you don't already have disadvantage. I'm not fond of it in the rogue's case, mostly for aesthetic reasons; it doesn't feel right to have rogues trading accuracy for damage. But it would be perfect for Power Attack.