The problem I see, is effective restraint of the dog once the threat is ended, or the dog attacking a 3rd party once his aggressive instincts are aroused.
I also have a rather protective dog. I carry 99.9943% of the time though, so I'm not likely to allow my dog to resolve a defensive threat rather than using the gun (if either level of force is required I'm not going to put my dog's safety at risk... I'll use $5 worth of ammo instead.)
My rule of thumb is that I'm never going to actually release my dog on someone, outside of my own property, though. If an aggressor gets close enough that my dog doesn't like him/her, then the aggressor deserves whatever they get. Stray inside of leash distance and attempt to hurt me or my dog, and I have no pity for you for the results you earn.
ETA: One last thing. I don't know about you, but my dog is my only household companion. He's a dear friend and I have no wish to risk his legal or physical safety by using him as a defensive weapon. I do my best to protect him from such situations because I love my dog. I'd be ashamed and unable to forgive myself if my lack of preparedness resulted in my dog acting to defend me, then getting put down by animal control afterwards.