The idea of using "B-list" companies makes me a bit disgusted honestly. Have we put ourselves on such pedestals that we use other people's valuable time as unknowing practice runs? I would never interview somewhere I was not interested in working.
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).
The same logic can be used when applying to college (a very similar process). You apply to a bunch of schools, some of which you don't want to go to. If you have your heart set on Stanford and you don't get in, you still need to go to college somewhere.
I guess the advantage of interviewing at companies that at first glance you wouldn't want to work at, means that sometimes you get convinced by talking to the great people there. I've joined at least one company that wasn't initially my first choice for this exact reason.
I've also cut an interview short once I realized it was a waste of time (H1b formality). If I'm not being taken seriously I won't take them seriously and I certainly won't waste my energy on fizzbuzz questions.