Oh I agree, but sometimes the shock value works. Children aren't thinking things through and when your response catches them off guard it can be a good thing. Letting my boy know that not only would I not take his side, but told him I would see the teacher got a raise for having to deal with his attitude, he realized the crap other kids told him was mostly bullshit.
Same thing with my friend with his kid threatening to call DSS. They had heard it from another kid and thought it was a magical get out of jail free card, but when called on it they folded.
I never punished my kid for his admission either. My setting the rules did its job and years later he thanked me because he had grown up and now realized how I actually protected him then from himself.