1) Prologues are info-dumps
- Readers are picking up a book to read A STORY, not hear about all the worldbuilding the author has done. Any worldbuilding or history that is important to the story can be interspersed within the story.
2) Prologues are flashbacks
- Ditto the worldbuilding comment above.
3) Prologues should just be re-named Chapter 1
- I can see this argument, but I think this should only be the case if main characters and storyline are introduced in the prologue.
4) Prologues are too long
- Anything more than a couple pages feels like too much of an investment for the reader.
Why have a prologue? What makes an "acceptable" prologue? A good prologue introduces the main (or one of the main) conflicts for the story arc before the story arc begins. Think of the prologue in Game of Thrones, where we meet the White Walkers. As we meet the rest of the cast throughout the book, we keep thinking about the prologue and the danger we saw there. While the rest of Westeros is playing king-of-the-mountain, we know eventually they will have to get it together to face the danger the White Walkers present.
Can't you do that in the first chapter? You could. So how do you know if the prologue you wrote for that story works? Cut it. Have someone read the story without it. Get their feedback. Then give them the prologue. Does it give them any new information? Does it change their feedback? No? Your story is good without it. Yes? Keep it.
Let's go back to our GOT example. Without it, the main conflict of the story seems to be the power struggle within the kingdom. We still know that the White Walkers are a danger, but the stakes are not as high because we haven't seen the danger. (A downside of the GOT prologue? It's nine pages long.)
How do you feel about prologues? Are there some that you hate? Any that you love? Have you ever put a prologue in any of your writing?