alternative one - I read a book and maybe lend it to a few people who might tell me whether they liked it or not. Then it sits on my bookshelf for the next 40 years.
alternative two - I read a book and write an online, searchable review, then give it to someone else, who may write an online, searchable review. Then, as I no longer have the book, when my friend comes up to me and asks for a book recommendation, instead of giving them the book to read, I actually give them the name of the book and author to go and read.
Plus, if bookcrossing is frowned upon like napster, then so should second-hand bookshops be. I don't understand how BXing is any different to resale. Particularly as with second-hand sellers, you can usually chase down a particular book. Bxing not so much.