It's a lot more fun to check the mail.
It's easier to try new authors and genres when there's no worry about making a poor buy when 'shopping.'
It's easier to pare down the collection when envisioning an excited recipient at the mailbox versus the bored clerk at the used book place whose job is to call treasured volumes 'worthless'.
It's anti-consumerism.
It's thousands of people trusting one another and you just can't get much cooler than that.
It's available to those who are without a Public Library.
Occasionally you may find a "treasure," a hard to find book that you'd love to read, or otherwise couldn't afford.
You know that the person on the other end has really wanted the book they asked for, so it won't wind up on the 'I'll get to that someday...' pile.
Your unwanted books which would only be worth pocket change to sell locally have new value, since you can trade them for books you really do want (plus shipping costs of course).
Many users donate Bookmooch points to charities that provide books to hospitalized children, inmates, schools, etc.
Books that haven't been released in your country can be gotten from all over the world.
It's encouraging globalisation.
It increases procrastination.

Reasons to Dislike Bookmooch:

It's encouraging globalisation.
It increases procrastination.
It's polluting through the transportation, especially with encouraging people to send abroad. Air transport is very polluting.
It reduces the appeal of lending books locally (you are better rewarded if you send a book an ocean away to a perfect stranger than if you just give it to your neighbour next door).
Penny pinchers should remember that, like that innocuous-seeming daily $3.50 latte, shipping costs add up to substantial amounts over time.
Like many good things on the internet, it's possible to waste quite a lot of time with Bookmooch--compulsively browsing for books to mooch, etc.
Your to-be-read pile will assume frightening proportions.

Reasons to Like Bookmooch