This is only my opinion, and I could be wrong. I've been wrong before...but I've also been right a lot more than I've been wrong (smile). I've also been doing this for a long time and I've seen a lot of things/trends come and go.
In the past five years or so I've seen authors creating book trailers that are actually very good. You can see these authors have worked hard on them, and I would imagine have spent good money on them as well.
But I always wonder whether or not book trailers work. I've obviously never done a book trailer and I don't see myself doing one in the near future. When I'm shopping for books without a recommendation, speaking as a reader, I'm more interested in reading a good solid book description, reader reviews, and a sample of the book I'm interesting in buying. Sometimes I buy with the process of elimination. In other words, I might not like the book description much. But if the reviews are interesting enough (with me they don't always have to be good reviews...I've bought more books I love based on bad reviews than I can count) and the sample pages from the book are good, I'll ignore the bad description. It's usually a two out of three deal with me...and it's not always about a bad book description.
I can honestly say that I've never been prompted to buy a book based on a book trailer. And I don't know many people who have either. I'm sure there are a few who do. But does that small group of people warrant the time and cost of producing a book trailer? I honestly don't know and there are no valid statistics to prove it one way or the other. So in this case I always trust my gut instinct...and experience...and listen to that little voice deep down.
Now, if you are an author like John Irving or Stephen King, I do enjoy book trailers that are actual interviews with the author. Who doesn't want to find out what a famous author is really like? There's a great interview out with John Irving's LAST NIGHT IN TWISTED RIVER. I watched it after I read the book. But notice how I worded this, I watched it after I read the book...it was an added bonus to see what Mr. Irving had to say. I would have bought and read the book whether he did the video or not. And, I don't think this can officially be classified as a book trailer. It is basically the author discussing the book, answering questions.
It would be great if there were some kind of meter that gave valid statistics about book trailers. But there isn't. And I don't think they help sell books or enhance the reading experience in any way. The reading experience isn't like the film experience, where a trailer helps people decide on whether or not they want to see the film.
The main factors to consider, which to me are huge factors, are time and money. I'm not a huge fan of vanity promotion. So far I haven't seen anyone who ever made a book trailer come out and honestly say the book trailer paid for itself and gave the author a lot of extra money in his or her own pocket. If they did, you can be damn sure all publishers would be doing them and the publisher would be footing the bills. And this is why authors need to think like business people all the time...the same way publishers think.