Used correctly, a prologue can add to a novel. The problem is that most prologues are nothing but back story or an info dump which adds nothing but a boring beginning.
Most inexperienced writers believe that the reader has to be told everything up front, or she won't understand.
Readers understand, though, and they are often bored to death, as well, by an unnecessary prologue.
You have just a few pages to grab the reader or that editor so you have to get their attention immediately and hold them through the whole novel.
If the prologue does that and the first chapter can't, then the prologue works. If it doesn't, cut it out and sprinkle the information as needed.