Well, there are many different ways to value a book I suppose. I would look at it from a teacher's standpoint and the book's relevance to today.
One aspect of the value of this book is that it still has the ability to capture kids' imaginations and draw students in. The universal appeal of the plot, kids stranded on a deserted island, has not diminshed at all over the past fifty years. Every student can look inward and think "What would I do in this situation?" There is huge potential for discussion and debate, even among the least motivated readers in the classroom.
I understand the question here to one of personal versus cultural value in regards to this piece of literature... Lord of the Flies has been an influential and widely read book, inspiring films and television shows as well as presenting a neatly rendered example where culture ends and humanity's animal nature begins.
To point to one example of a show inspired by this novel, created fifty years after the book was first published, we can look at the television series LOST. This show harkens directly to many of the themes and narrative elements of Lord of the Flies.
That Lord of the Flies has lasted the test of time attests to its value. Golding's allegory teaches the reader about life, about "the evil that men do" as Shakespeare wrote in Julius Caesar. He forces the reader to recognize the innate evil and cruelty that is in man, an instruction that the civilized naval officer in an uncivilized war fails to recognize.
Value is in the eye of the beholder. The book is rich in symbolism, has well-drawn characters, and has an interesting plot. You could also argue that the book is strong thematically, giving us a warning about innate human nature and our society.
I, too, am unsure about the meaning of the question. I would think that the value of the book is that it asks us to think about what our human nature is and what we would act like in a "state of nature." This challenges us to reflect on whether Golding's view is true and, if it is, what the implications are for us and for our society.
But I'm still not sure what you mean by no value vs. value. Can you explain a bit more what you mean?
I'm not quite sure how to understand this question. Are you interested in the moral values expressed in the book or how a reader might learn something about the nature of morality by studying the book? Or are you interested in the notion that the value of the experience of reading it comes as you reflect upon the situation of the book after you have finished it rather than during the reading experience?
One approach you might find useful is reader response theory, i.e. that the value of the Lord of the Flies lies in the way, by the end of the story, you respond to the social commentary in the book and ask yourself whether young people you know, freed of external adult restraint, would, in fact, descend into savagery and murder. In that way, your payoff, as it were, for time spent reading the book comes in reflection on its wider implications for how you think about civilization.