The other day I was talking with a friend of mine about why one does or does not acknowledge others for their help in one's papers or books. My friend, who does research in the field of psychology, was complaining because he had read in detail, and commented on, a couple of papers by two of his co-workers in his research institute, but when he read the published papers there were no acknowledgments for his help. He was a little angry because he had made some (in his view, important) suggestions about how to interpret the experiments his co-workers had conducted. He asked me what the usual practice is in philosophy, because in psychology one generally thanks others for their comments or criticisms. I told him that, as far as I know, most of the time in philosophy papers and books one does devote a note or a page to thank others, but that this does not always happen. I remember I reviewed a book last year in which there were no acknowledgements whatsoever, even though I suspect that the author received extensive feedback from others. What is the reason? Does the author want all the credit for his/her work? Probably, but there may other reasons as well... I don't think that the lack of acknowledgements in the case at issue is that serious, though I agree that they are a polite way of thanking a person for his time and help.
There are other, related cases: when one has read another person's work or has heard him/her at a conference and uses some of his/her ideas without acknowledging them, which seems quite dishonest (a related case is reported by M. Forster in Kant and Skepticism, p. 132, n.8). Also, sometimes one does not thank the referee of a journal for his/her suggestions or criticisms, which is not very polite either. If one wants to be radical, one should even acknowledge the comments of a referee of a journal which hasn't accepted one's paper - but it would be extremely ridiculous to write something like "I thank the anonymous referee of this journal for his useful comments, and also the referee for the journal X, which rejected the paper, for his helpful criticisms". Finally, one could thank native speakers for correcting the grammar of the papers one writes in other languages - which is not a common practice either, but which I have started to adopt to thank friends for their help.