This Thing You Call Aggro.

[savaughn]

I agree with this for most cases.  Of course, I can think of a good exception off the top of my head. ;)
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I don't there is an exception, per se. Off tanking is off tanking. Something is taking damage other than the main tank. The only real difference is that a pet heals rapidly after combat if it doesn't die, so using a pet can save the healer mana if the mob isn't going to kill it in time.

The real risk with using the pet is that you need to remember to put the pet back onto the main /assist target, or you're disbursing your focused fire. Sending a pet and leaving it means you kill your main target slower, which means the party takes more overall damage. This counteracts any possible benefit the off-tanking would have had except in strange instances (like a pet off-tanking an AoE mob like in your example).