Prologues: do we really need them?

Today I was reading an interesting discussion on a Google+ group. An editor was asking whether you should consider the first chapter of a book to be a prologue if it is set 20 years before what happens in chapter 2.

That made me curious as I've written a novel whose first chapter is set 20 years in the past, but it is not a prologue to me.
The novel I'm talking about is a crime thriller I've written during the past NaNoWriMo, titled "Il mentore" ("The Mentor"). The story starts in the past, in 1994, when the two main characters met for the first time. Then the story skips to 2014, because the 20 years in the middle are not important for the story itself, but one of the two main characters continues referring to what happened in the first chapter in her blog (which is reported here and there in the book). That first chapter, all the blog posts and one scene at the end of the novel are written from the POV of this character, in present tense and first person, while all the other scenes are from the POV (third person) of another character (most of them are from the POV of the real protagonist).
The whole book is strictly connected to the facts in the first chapter. So it is not backstory nor a prologue, it's the first chapter of my book, where everything starts.
Actually I often start a book with a scene set in a different time than the one of the following chapters, sometimes it's in the past sometimes it's in the future. Only because it is in the past that does not mean it must be a prologue.

In my opinion a prologue is something happened in the past which helps to define a situation or a character in the main story, but it is not crucial in its development.
I cannot tell you exactly what's written in the prologue of my book, because it's still unpublished, but I can say that it narrates an episode in the past of the characters, whose details are used by a murderer along the novel, so it is definitely crucial.

It must be said that the use of a prologue is often considered not necessary. Most books about self-editing suggest not to use it at all, but rather add some backstory or flashbacks here and there in the novel for describing the facts. This way you can bring the reader directly in the middle of action (in media res) and avoid to lose them after reading the first sentences.
I definitely agree with this. I love starting my stories in media res, actually I try to do that in most scenes.
I also prefer to read books written this way, without a prologue and which throw you directly into the story.

But what do you think about prologues? Do you like to write them? Do you like to read them?