I can think of three reasons for interviewing at a "B-list" company that have nothing to do with putting oneself on a pedestal.

1) You might actually like the work/environment/personal growth opportunities/company health at the "B-list" company once you see it in person. Admit it: your initial classification involved a healthy amount of guesswork and is probably not fantastically accurate.

2) A stupid mistake in your A-list interview hurts both you and the A-list company, potentially dramatically so. You help both them and yourself by practicing.

3) Your increased bargaining power decreases the chance the A-list company lowballs your salary, shooting both you and themselves in the foot (hiring your replacement is expensive, lost productivity due to dissatisfaction is expensive, envy-poisoned work relationships are expensive).