There are cases when lethal force is not justified in regard to the threat. Brandishing the weapon can lay in a red-hot legal area.
Like a bigger, stronger than me, guy that is in my face yelling threats. I can feel it's rage in the precipitations... Is that a deadly threat yet? I don't know if I can explain why I shoot him. But a non-lethal way to get out of that situation is easier to explain. Legally fight any eventual charges of assault versus homicide ones...
Again, that's just me. Some people can impose by their size only (Chuck Norris types).
No, that's not what I meant - what I meant was that the spare magazine is ostensibly a backup piece of the lethal-force equation: if that's the case, then substituting its space/weight for the stun device that you'd cited isn't a true 1:1 argument.