I have only wanted to buy one book because of a trailer. The problem is they aren't really where books are, and if they are, they are out of the way. The one I did buy, I couldn't help but I wasn't looking for it, just ran into in youtube. It seemed interesting. I don't like the one's with only the cover and lots of dialogue, but at least they are getting their names out there.
Some of the funnest one's I've seen aren't professional at all and my friends simply made them for fun. One even shared one of their so-so reviews on it, and labeled it with a curse word instead of anonymous. I laughed my butt off. Really, they are probably more for authors but that might change as technology comes around to match wits. But then, would people be disappointed with the lack of graphics and excitement in the book?
It's an iffy field, too new (some people still don't understand what a book trailer is) and nothing is really certain. I make my own more for fun, but I wouldn't shell out hundreds of bucks for a fancy trailer.