Back in the day, I used to be anti-prologue. I thought whatever was in a prologue should be worked in the body of the text. I was quite certain, as only youth can be, about this.
However, I've gotten over it, because I've discovered that sometimes, a prologue can save a writer a lot of grief. I tend to write heroes especially who are the strong, silent type. They all have issues and problems in their past that have made them this way, and that means they aren't about to "open up" any time soon about those pasts. Eventually, of course, they do, and to the heroine. Unfortunately, it seemed a lot of critics had already passed judgement on the poor boys before they got to that part, and by the time the hero explained or described his past, it was too late. They were already toast in the critic's eye and apparently beyond redemption. I don't fault those readers for making too hasty a judgement. I failed by not making my characters sympathetic enough before the Big Revelation. Since I knew their past, I already saw them as heroic, so didn't realize the need to make it clear for people who didn't know the character as well as I did.
This type of hero really benefits from a prologue. By showing a scene from his past, the reader is much more inclined to feel sympathetic toward him, or at least understand why he might not be too keen to reveal certain elements of his past that explain his actions in the present. Instead of coming across as simply stubborn or arrogant, the reader has a clue as to why he's behaving that way, and is more willing to cut him some slack.
With the new book, MY LORD'S DESIRE, I'm doing something a little bit different -- the prologue is about the heroine and her sisters. I want to show the reader why she doesn't want to get married, to have them understand what in her past has made her so determined not to. I feared she would come across as too cold, too "ice queen", too unsympathetic to the reader, if I didn't.
I wrote the prologue this morning. I didn't intend to -- I was supposed to be writing a "dear reader" letter for HERS TO DESIRE. But I had the ideas bubbling and decided, what the heck. Write it. I'll do the "dear reader" letter later. And that's what I'm off to do right now.