Spirit of Barbs is, much like the Thorns aura the Paladin uses, all but useless. It only reflects melee physical damage, and only damage taken. While this works excellently for the Paladin at early levels, the druid must wait until lvl 30 to get this skill, and while it could possibly be useful in Act 5, by the time the player enters Nightmare the skill will become more and more useless.
The problem lies in the game's scaling. Monsters both increase their damage and life as the game progresses, but their life increases at a much greater rate. So while in Normal the monster may deal 10% of it's life in damage, the same monster might only eal 3% of its life in damage in Hell.
Conversively, the player increases their damage output faster than their life. This creates a deadly discrepancy. Since Spirit of Barbs depends players/minions actually getting hurt, they will have to risk their lower health pools constantly in order to deal damage, and that damage makes less and less of a dent into monster hit points as the game progresses. Eventually the danger versus the payoff becomes rather slim, which in turn is why almost all druids instead focus on one of the other two spririts: Oak Sage or Heart of Wolverine.