I will venture this:
respecting someone who carries a gun, and either through personality characteristics or situational cues, may (I use may to describe the choice of the officer, not the justification of their action) use it on you is not a matter of necessity (re: you shouldn't just respect a cop because they are a cop) but of pragmatism (re: we all know cops abuse power when people lip off at them).
This whole debate seems to be waged between 2 sides who have a conception of how a perfect world would work. Police in a perfect world never would abuse their authority. Citizens will cooperate in a perfect world. In the world we live in, neither of those things happen all the time.
And I'll throw in with Bardock. My life is made immeasurably better by the existence of police (if not by the existence of some laws) but that benefit is not proof of its necessity. Were we to qualify it and say "police are necessary for..." there are many things I would have to say do rely on police, but they are not just abstractly necessary.