Heart of Wolverine is very similar to the Fanaticism aura the Paladin uses, but with some differences. This skill provides equal damage and attack rating boosts to all, druid and party members alike, while Fanaticism only increases party damage, but instead buffs the Paladin with increased damage, attack rating and attack speed.
Heart of Wolverine is quite useful and melee druids probably wish to focus on this one rather than Oak Sage, since a druid can only summon one spirit such as this at any given time. It stacks with Fanaticism, so a Paladin and Druid with these two skills can provide massive boosts to the party.