Now of course, there's no right or wrong here. Whether a prologue improves a story or not is dependent on the story. And we can't generalize, except to say that some types of books (suspense, maybe) often have prologues, but others (like romances) seldom, and that probably has something to do about the major motivation or conflict being past (in prologuey books) or present (in non-pro books). However, let's consider why one group (the publishing pros) tend to group over there with "no prologue" mindsets, and the other (authors) often are pretty adamant about writing and keeping the prologue.
I don't have any real bias against prologues, but I agree with Theresa: We've read many really bad and/or unnecessary prologues. This doesn't mean that your prologue is bad or unnecessary, but all those authors we're thinking of also thought their prologue was just great and absolutely essential. (There's nothing like being an editor to make you question everything.)
So I'm going to give the arguments against the use of prologues, and if you like prologues (as a reader or writer), maybe you can give the arguments for using them. But of necessity my points are going to be why prologues are often not good openers for their books, and the pro-prologue argument might be more individual-- that THIS prologue served this good purpose, that it was right in this case. That is, I can generalize, but you probably can't.
Easy Answer Time
There are the bad prologues, but in some way, those aren't much of a problem, because usually they lead into bad books. So it's not like the prologues ruin the book. They just preview the book-- bad prologue-> bad book.
That makes it an easy decision for the editor or agent or reader. "Not worth the time to read"-- and the prologue isn't the problem. The first chapter would create the same verdict.
But we don't have to waste a lot of time on bad books, right? That's not what you're aiming for. :)
A Tougher Dilemma-- Good prologue, just not maybe for this book
However, occasionally there will be a prologue that isn't so simple. Like it's not a bad prologue-- well-written, tautly paced-- but it opens into a different sort of book. It doesn't seem to match the book or connect with the story in either event or tone. You know, there's a prologue about a child huddling in a closet in Manhattan, terrified, and she's discovered by a murderer and killed. Then Chapter One starts in Minnesota, where parents are dropping off a young man for his second year at the state university. Hmm. Was he the murderer? Was his father? Was the victim his sister? Am I supposed to be happy he's greeting all his old buddies and bumping fists with his roommate, or still traumatized at that child killing? I don't know. But the disconnect might raise questions you don't want me to voice, like "Did the author have two different openings to two different stories and just smush them together so as not to waste any words?"
Or it's perfectly good, well-written, all that, but is only there to set up the backstory of a character. Sometimes when I've questioned authors, why the prologue? they have reasons, like "I need to show why even when she's in the big city, she worries about gossip. So I start with a prologue when she's growing up a small-town and her grandmother tells her that everything she does is grist for the gossip mill." There are always reasons, I understand. You can justify everything. But is that the best way to get this across? Isn't it possible just to show the character in the big city, in a restaurant, acting surreptitious and confessing that she's sure the gossip columnists will have a field day with her sending back her undercooked salmon? Action that happens in the story is likely to be more vivid and plot-changing that something that happened years ago.
Maybe-- and this is quite important in some genres, particularly romance-- the prologue focuses the reader on the past, when the present is what ought to be most important. After all, if you have a prologue, you are starting off with the assumption that the event in the prologue is really important, and that event and that mood will preoccupy the reader as she goes on and reads about this couple meeting in the present. The reader might experience the emotional residue from the prologue and have trouble getting in the right mood for a romance, or be too preoccupied with the past ("Isn't anyone going to solve that kid's murder? It's been ten years!") to concentrate on the present day of the book.
For me, the most important question is-- What are you giving up to have this prologue? a lot of authors just put them in there without thinking of possible tradeoffs. Are they diminishing the suspense by telling about some important event that should be discovered in the book? Are they losing the reader by starting too far back in time, or by launching into an event so grim that the reader puts the book down without buying it ("too depressing")? Maybe the prologue anticipates something the character must find out, and so deprives the reader of the enjoyment of discovering it along with the character.
Or maybe it's perfect. Maybe it's right for the book, right for the genre, right for the characters, right for the readers. But how do you know? Maybe you needed to write that prologue in order to write the book, but should you keep it?
A few questions, just to get us thinking:
1) Is the prologue an event or what? If it's not an actual event that took place in the past (like it's a slice of small-town life), could it be Chapter 1?
2) Is this an important event? Why? What do you want the reader to be thinking or speculating about the prologue event as the book-proper opens in Chapter 1?
3) Does the event or whatever have to be in the past? Can it be incorporated into Chapter 1 (like a big fight with her mom-- could it happen on the phone in the chapter)?
4) What is the effect you mean to create for the reader? If you want the reader to ask a question, what is the question? If you mean to set a mood, what is the mood? And of course-- what is the effect of having that question or mood as she starts reading Chapter 1?
5) If there is a question in the prologue (an unsolved mystery, a secret), is it somehow resolved in the book? When? How?
6) What are the connectors from the prologue to Chapter 1 that let the reader know he's in the right book? That is, are there any commonalities (of setting, of character) that you can use to assure the reader (if you want to) that the prologue does connect to this opening? If you don't want the reader to know that the child who is threatened in the prologue is the bride in Chapter 1, how can you create some subtle connection so the reader doesn't wonder if the publisher bound the prologue of a suspense novel to the opening of a romantic comedy?
7) What are the tradeoffs? What are you giving up by previewing or postviewing something from the past in the prologue? Be honest here and not defensive. There are always tradeoffs! That doesn't mean you're wrong to want a prologue, only that you should be able to know what you're losing by gaining what you gain.
8) And with every opening, the question is, is this the best place to open the book? What issues, conflicts, situations does the prologue set up that will be developed by the book?
What are questions do you think would be helpful in evaluating the usefulness of a prologue?
Also, if you've written or read a prologue that works for the book, can you analyze why this one works? Also the tradeoffs the author had to deal with?