“Tell me of what value was it for Huckabee to wonder out loud about another candidate’s religion?”
Your taboos are bizarre, two religious authors discussing something as simple as religion on a two hour drive is acceptable in my view. Yes that includes a Mormon question arising, especially when a real live Mormon is in the race.
In politics it's called exposing yourself for unnecessary reasons. The fact is not that one can't discuss said subject but the wisdom one shows in choosing what they do or do not discuss/answer. He not only didn't show wisdom, he was dishonest about his intent (just like the Iowa news conference with the negative ad) and then even what he actually had said when called on it.
The bottomline is that you can't answer the question of what benefit it was to ask his rhetorical question no matter if the interview is casual or not.
Just a tip: if you are running for President EVERY word you say can/will be used to judge you in the court of public opinion. Mike's playing dump while being interviewed was dishonest curiosity.