Her answer should be the obvious one. Of course it was imperative to defeat slavery - that is, after all, what the Republican Party was founded on. But that doesn't mean every by-product of the Civil War was positive. The reality is quite different. It simply means that reality can provide an example that undermines the efficacy of a principle.
Life causes problems for theories all the time.
Those traditions were steamrolled onto the ash heap of history by the Civil War even more than states' rights were. On paper, states' rights and racism are apples and oranges. In principle, the federal government could easily have been pro-slavery, and the abolitionist states could have been the ones seeking to break away from tyrannical system that respected human bondage. Consider the histories of Germany, France, Russia, Cambodia, China and much of Africa. Centralized governments tend to be the enemies of freedom, while countries that respect local autonomy and self-government are more likely to respect civil liberties.