I've watched the evolution of a few of these things. My experience is that even if people aren't trying to be jerks then the quality of books inevitably goes down over time as people take the things they find interesting and replace them with things they find less interesting. You might think that since different people find different things interesting that this might work out in the end, but it doesn't seem to.

Add to this the fact that some people are just jerks and will take books without replacing them, plus a bunch of people who see this as an opportunity to promote either their self-published books or their own political opinions, and you get a bookshelf rapidly being denuded of popular books and filled up with political tracts, dated "how to" books and obscure genre novels from the 1970s.

Compare this to, say, an ordinary public library, which allows you to freely borrow any one of many thousands of well-catalogued books as long as you return it in a few weeks.