I haven't been to the arena yet, but that definitely makes sense as to why someone would use a Shadow tank for that.

I've heard that your main pet gets attacked most, but for some reason I've noticed that when I tried leveling a shadow tank it really seemed to be my other 2 pets that were taking all the damage (and not from attacks that were hitting everyone) maybe that was just bad luck on my part.

Thanks for all the extra info ^.^