Summon Spirit Wolf is the second useful summoning skill the Druid has, although the usefulness of it is undebated. Spirit Wolves are weaker than Skeletons summoned by the Necromancer, and a Summoning Druid never reaches quite the power that a Summonancer does. With this in mind, most druids simply focus on Summon Grizzly to act as a tank and all but ignore the wolves, but which is unwise.
Spirit Wolves, if focused on however, are synergistic with Summon Dire Wolf and Summon Grizzly, providing passive bonuses to both their own and the other summons' Attack Rating and Defense by the same percentage as seen above in the table. So the more points spent in this skill, the better Dire Wolves and Grizzlys will be at hitting their enemies and avoid getting hit.
Dire Wolves in turn provide the Spirit Wolves with increased life, and the Grizzly them with damage, making it absolutely vital for a Summoning druid to spend as many points as possible into his summons.