I have found a valid and potentially powerful use for Tornado. It's challenging to pull off, but when done successfully the Tornado becomes in essence an AoE hold.
The premise that makes this possible is wrapped up in how Tornado chooses it's next target. When you first create a Tornado, it (apparently) creates a list of all mobs that are either in aggro range or already aggroed on you. When it switches targets, it will choose a mob from this list at random (whether it ran away or is still nearby).
To pull this off requires the use of Hurricane (Gale can make this easier, but is not absolutely necessary) and - most importantly - a corner. Specifically, a corner as close to 90 degrees as possible. Having a ceiling of some kind is even better, to prevent the mobs from being blown up and away.
The basic idea is that, if you can get all the mobs involved in the melee into the corner (or reasonably close) and keep them from running out with Hurricane before you put the Tornado so that the mobs are between the corner and the Tornado, then all the mobs on the Tornado's list will be blown into the exact same spot - the corner. Therefore, when it switches targets, the new target will be in the same spot as the old, so it will keep attacking the same place. Keeping Hurricane up and standing so its knockback is directed into the corner will ensure that any mobs knocked up and away by the Tornado will go into the corner rather than away from it.
The end result of this will be the sight of the legs of the mobs hovering 2/3 of the way above the floor and sticking out of the wall for the duration of the Tornado's lifetime. During that time, you can unload all your AoE attacks on them.
The biggest challenge to using this tactic is getting all the mobs into position before throwing Tornado - if even one gets past you, there will be a chance that he'll be the mob Tornado picks to go after next, dropping all the rest as it leaves the corner to go after him. Using Hurricane only to set this up is tricky (here's where Gale would be useful). Don't forget to also use good old fashioned tanker-style positioning tricks to get all the mobs in the right spot.
As an aside, I've repeatedly encountered a bug with Hurricane. It's inconsistant, but on occasion the logic used by the game to figure out the direction a mob should be knocked back from me will be reversed in value - meaning a mob in front of me will go towards and past me instead of being knocked directly away from me. (I've already bugged it.)
