a) If they are good, they make the author look good
b) If they are crappy, the reader assumes the book is also crappy
c) There is no such thing as a crappy fan-created trailer--even if it's made with Poser, remedial Photoshop, or a shaking iPhone, it's still done by a fan. They're a visual 5-star review.
Now that I've stated the obvious. I'll atone for my sins with three examples of very well done book trailers.
1. Lori Foster -- Contemporary Romance
Lori makes mini-movie book trailers for all of her books. She does them for two reasons: she likes them and the fans like them. If you ever attend the annual Reader Author Get Together she sponsors, you will hear first-hand many, many times just how much her readers enjoy those trailers.
Production Company: Onion Creek Productions
2. Holly Black -- Fantasy for Kids & Teens
Animation + creepy nursery rhyme sung by kids = 30 seconds of total win.
Production Company: Nice Studios
3. Tiffany Schmidt -- YA
Text + Imagery + Music = A synopsis that told me so much more than the title or the cover alone conveyed. Hello, Once Upon A Crime Family where fairy tales and the mafia collide?
Production Company: Flatline-Films