You better hope your dog is more of a killer than the one charging you, chances are it's not. and even if it were, would you risk your dog's life to save your skin when all you'd need to do is put a jhp in ol' sparky?
That is the whole point. Rather than kill the animal prior to it demonstrating it is actualy attacking and getting into trouble, the very brief attack on your own animal shows it is in fact a danger, at which point you can proceed to deal with the animal unhindered.
Whether you have a gun, a blunt or sharp object, or some fingers ready to remove some eyeballs, it is going to be much easier when you have your own animal on your side.'
That 90 pound pit is not going to do much serious damage to your 40 pound animal in the few seconds it takes for you to unholster your weapon and put it down. You can attempt to break up the fight first if you are in a really unfriendly state, and put the animal down when it turns on you as it no doubt will.
Either way you are in a better position than without your animal. Plus if it is injured you can get it the best care and take care of it. If you are injured your animal sure is not going to be able to take care of you or pay the bills.
I would think your animal getting bitten sure beats you, your wife, or your kids struggling to get by because you cannot work with your injury.
The life expectancy of a large dog is usualy 6-12 years. A humans life expectancy is around 70 years. Who is going to be dealing with problems arising from injury longer, and who's injury is going to effect more individuals?
Also who can prevent more injury from occuring while the other one is being attacked? You or your dog? I think I would save my animal faster than my animal could save me from another large animal, and my attacks are going to inflict a lot more damage than the attacks from my animal, I use weapons.