It's a way of framing an argument that discourages rational discussion of the actual merits of each side. It's not whether or not I agree with that value judgement that matters; it's that assigning those kinds of judgement is often a way to preclude any deeper or more nuanced discussion.

That's a fair point, and it's easy to get caught in the trap of discussing things in such absolute terms.

I aim for clarity (often missing it entirely), and using absolute terms such as 'good' and 'evil' reduces clarity. I'm glad that I found this comment.