I'll try to answer, although I am not sure what your sentence about the starting premise that exploration exists and is worthwhile is referring to exactly.
There are background interviews, and there are interviews done to produce stories for publication. There are also casual talks with sources, social meetings, and other kinds of contacts that may or may not result in something getting published. There are interviews that produce Q&A stories, where most of the interview is published, and there are off-the-record interviews where nothing that gets published can be attributed to the interview subject.
In other words, no, every interview does not have to be for a published story. However, as a subject or source, I would wonder why I was answering questions if there was no intent to produce (eventually) a story. Also, more than one interview (with maybe follow up questions later) is unusual -- unless the subject has time on their hands and likes talking about the topic of the interview.
If I were planning a book, I probably would set up a series of interviews, and would explain that I was researching the topic and was not yet sure how the material would be used. I would follow up with the interviewees when I had a more concrete outline to be sure they were comfortable with the scope of their participation.
It is impolite to waste people's time, of course. But if you are up front and honest about your intentions, you will avoid misunderstanding. You can certainly approach a source and request an interview because you want to learn more about a topic or gain background information that helps you decide if there is a story to be written. Student journalists often have to request interviews and explain that the result is expected to be a class paper, not a widely published article. People are often generous with their time to help students educate themselves.
To show respect to your subjects, be prepared, be on time, don't ask for more time than you need -- 45 minutes to an hour is about as much as you can ask of busy people. If you really need to have a series of interviews with the same people, you have to make it convenient for them and somehow worth their time -- not with compensation, but by helping foster communication in which they are interested.