While I must admit that I do have a less than stellar
relationship with most of my ex-boyfriends, or they have simply dropped off the face of the earth, I agree
that having exes in your life can be a good thing.
They remind you of the things you didn't want in a relationship, and keep you from getting caught in that trap again. Now that you know what you don't want, seeing the ex again reminds you again of how much you disliked that character trait. It makes it easier to find people that you do want to be with if you can identify the things that you really want and don't want from a relationship.
They know a lot of things about you that other people might not, including close friends, so they can be invaluable for advice. Of course, this depends on if you are on good terms with the ex or not. They've seen you naked. They know the dirty little secrets that a lot of people don't know. When someone knows things like this about you, you have to realize that their advice could be right on the money. I know that's true of a couple of exes that I talk to regularly. They almost always know what to say that will help, and will call me on any bullshit that I might be using as excuses.
I'm glad that as an adult, I've generally been able to remain on good terms with the men in my life that I had an intimate relationship with. Its not easy, especially if you end up hurt, but it can definitely be a good thing.