That's a silly rule. Prologues are like any other literary device, and can be used well or ill. I used one in my gothic mystery novel The Irresistible Summons, completely unrepentantly, as I believe it creates a vital instant connection with the protagonist. Without it, the story would take longer for the reader to connect with her. My editor at the publisher concerned agreed, incidentally. So mercifully I didn't have to have any hill-I-will-die-on debate with her on the matter.