There's a bit from Herbert Spencer that some of you may know: "There is a principle which is a bar against all information, which is proof against all arguments and which cannot fail to keep a man in everlasting ignorance—that principle is contempt prior to investigation."
Now contempt, while largely and correctly acknowledged as a negative emotion, can also be a powerful weapon in a critic's quiver. It's always a double-edged sword, though. To Pauline Kael's admirers, her contempt was withering, magisterial; to her detractors, it was petty, spiteful. Those diverging perspectives remain pretty constant, almost a decade after her passing.
I'd think we all agree, though, that prior to unleashing one's contempt, a critic ought to do what lawyers call due diligence, and have a pretty thorough understanding of what they're lashing out at.