One point no one has made in this thread, so I'll go ahead and make it, is that registering copyright not only makes it easier to sue someone who infringes on you; it can help protect you when someone else claims you are violating their copyright. Say you write a book that makes less than $5,000, but your plot, characters, and even some direct passages show up in someone else's book a few months later. You don't even notice. That book becomes a big hit, and its story is made into a big budget Hollywood movie, with action figures, themed Happy Meals, etc.. Just before the movie premiers, the production company--whose lawyers comb the Earth in search of threats--sues you for violating their valuable copyright. Yes, you technically "own" the copyright by virtue of publishing earlier. Should you hire them for way more than the $5,000 you originally made, high-priced lawyers can ultimately help you prove it, but your case would be much stronger with an earlier copyright in hand. In fact, it will help you get a big chunk of the movie and pay for the high-priced lawyers to boot.
My advice: if you think the book will earn more than $50, spend $35 to register the copyright. It's worth it for the piece of mind.
Legal disclaimer: I am not a lawyer, but I play one in the classroom, where for over 20 years I have taught a college course entitled "Media Law and Ethics." (Yes, I know that's an oxymoron.)
The purpose of copyrighting is to protect yourself. Will you likely use it? No. Is it copyrighted once you write it? Yes. Should you copyright your work. Yes, to answer your question, I do think it is worth it.