My guesses as to the purpose of "Call of the Beast":
1. It targets the WILL defense, which the other 7 Druid At-Will Attacks do not; so it's more useful against Zombies and other low-will attackers.
That I pegged.
2. It deprives any targets it hits of the ability to gain Combat Advantage; this is a salient part of the Controller role, because it reduces the combat effectiveness of the enemy.
This is a bit of a mixed bag for me.
Call of the Beast hits all creatures in the area. So if you
don't want to zap your allies, this means you use it when the enemies are Over There, not Over Here next to your friends. So, the only chance the guys Over There will have combat advantage against your friends Over Here is if it's a surprise round (which isn't useful for the power), the enemy is hidden (good luck hitting them), or your friends are down on the ground (under a status condition, prone, etc). So that limits the usefulness of it right there.
The only other instance that comes to mind is in melee with your allies, such as flanks, or again, status effect + melee. Which means that you're going to have to hit your friends in order to stop the monsters from getting CA. And if your friends can't attack their nearest enemy (for whatever reason), then they're taking damage.
Not to mention that the only monsters where CA is all that significant is the lurker and skirmisher. For other monster roles, the CA is just giving a +2 to hit.
It's far too situational for my tastes, for an At-Will. I can't see even using it every fight, because its usefulness depends on the makeup of the monsters you're fighting and their place on the battlefield, and your allies positions. It's a headache.
3. It imposes a penalty that the enemies know about, which only takes effect on hit targets that do not attack the nearest of your allies; this also reduces the combat effectiveness of the enemy, because they now take damage if they do not attack the nearest Fighter or Paladin or Ranger or Rogue or Warlord instead of surging en masse to attack the Cleric or the Druid or the Wizard -- i.e. the notably squishy Leaders and Controllers.
Of course, to do that they have to rush past the defensive line. If they're at your flank, or the nearest allies are not your defensive line, then you're even encouraging the enemy to go after the squishies. (Also, I think the strikers are squishier than leaders; the latter has better armor).