Every now and then someone in a writing forum asks about prologues. Are the forbidden? Are they a good idea? The replies break down into: prologues suck, I never read them, simply fold the back story into the rest of the novel; or it's part of the book so I read them; or the ever non-helpful, if it works for the story then it's OK.

I'm of the opinion that the author put it in and it may contain information that I would find useful and it's usually a small part of the story so I read it. From a writer's perspective I haven't had an opportunity to need one.

To me "prologue" is simply a label for a chapter sized chunk of text at the beginning of the story. The label implies that it is written in a different time, place or voice than the main story. There are a few times where a prologue is very useful. Such as, to set the background for a historical story that is not commonly known, or to recount the previous events in a series. I wish Jim Butcher would do that in his Dresden Files series. It's not that Butcher doesn't write well but after ten or so novels I'm a bit tired of hearing about the histories of the characters every time.

In the end it is a style issue. And styles change.