Dire Wolves are much more powerful than the Spirit Wolves, however they have some serious drawbacks. The Druid can only summon three at once, compared to the more than 10 skeletons the Necromancer easily puts out. While one might think that the power of the Dire Wolves would offset this, and with enough synergies they will become quite powerful, three wolves just isn't very much and can't hold dozens of enemies at bay.
Dire Wolves get increases defense and attack rating from Spirit Wolves, and damage from the Grizzly, and provide increased life for the other summons as a passive bonus.