Although this causes no extra threat specifically, doing normal weapon damage to four enemies at once can go a long way to keeping aggro in a cluster of mobs. Also, each individual hit has the normal chance of going critical. Two or more criticals from a large two-hander can make a decisive difference to the average cluster of mobs.
The rage cost is a little on the steep side, so this is best used in conjunction with [Berserker Rage] to compensate. Note that unlike most warrior abilities, this one is truly AoE and will hit targets directly behind the warrior.
When using two one-handed weapons, it hits with both weapons.
Be careful of crowd control with whirlwind however. You shouldn't be breaking your teammates sheep/shackle/seduce.
This attack also has an additional use in PvP - due to the fact it can hit anyone in melee range regardless of where they are attacking you from, it can be used to teach a lesson to that irritating Rogue who is constantly backstabbing you.
With its 8 yard range, it can hit enemies in your dead zone (Allows you to hit enemies too close for you to intercept / charge, yet still far enough that if they're out of your melee range, you can hit them).
Once you get [Titan's Grip] (Fury spec), Whirlwind causes very high damage, making it worth fitting in all your rotations, even single target. After all, causing instant damage with double two-handers is always good. (NOTE: While this **MAY** be true while leveling and you have limited skills, it is in no way the case once you have the majority of your skill set)