This may be due to the fact that gamers can't get enough use out of the books to justify the cost, or that the book is somehow substandard in terms of overall quality (i.e. typos, redundant information, padding, etc.). I have books which I have not used enough to justify my buying them, but I would say that has more to do with the sorts of campaigns I run versus the overall usefulness of the book.
I would say to gamers before buying: think about your campaign and whether the book in question will serve your vision in the best possible way. Don't be sucked into the 'everyone else has it, so I have to buy it too' trap. There's no shame in not owning something, even if everyone else seems to be talking about it. There's only shame in wasting your money on something you have no use for.
I will buy far fewer books when the inevitable 4e leers its ugly head, perhaps no more than the 3 core books we all rely upon.