FlyerJohn, silly me! I forgot about the dual lights. I liked your other answers as well (day/night, MEL, ect). However I was under the impression this type of question was asked to test mainly your integrity/safety attitude/conflict management, not so much systems knowledge (I might be wrong though). Don't get me wrong, I think you brought up some good points, and I would probably answer in your same way if asked that type of question. I'm sure asking questions like yours shows critical thinking and proactive problem-solving, but what if they were interested in knowing how you would handle the conflict with your captain?
If that is the case, they will answer all your questions saying that BOTH navs are out, it is night, it can't be repaired that night, it can't be MEL, ect. In other words they put you in a situation where you either break the FARs and go with what the captain says, or stand up to the captain and tell him its a no-go.
Obviously, you don't want to break FARs, or admit you would at an interview....

How would you handle the conflict with the captain if he was completely motivated to go? What if you cannot convince him be talking to him?
What do you do? Refuse to board the aircraft? Call company ops and denounce the matter?

I know this is a far-fetched scenaro...but reading some gouges it lookes like they are not that uncommon.

thanks to all that replied!