It’s very constructive. Soviets and Nazis both committed genocide. But anyone with a shred of morality should have no hesitation, if it’s down to the wire and only the two of them to choose from, which one should side with. The Soviets, if that wasn’t clear. The Soviets, awful as they were, and even though they were quite literally genocidal - on multiple occasions, on a massive scale - are very much the lesser evil compared to the Nazis. The idea that there is no ‘lesser evil’ when genocide is involved on both sides as a matter of general principle would necessarily imply that the Soviets and the Nazis are equally evil, which is fundamentally absurd on its face.
It’s a hollow argument trotted out to make “LIBERALS BAD FOR FOREIGN POLICY” sound more profound; it’s not a sincerely held principle, and the inability to answer that challenge displays that.