Are nightmares something to be avoided?

Are they a problem in general, or only too many of them? How many is too many? Do they do us any good? If so, what is good about them? Have you, personally, ever had problems with nightmares? If so, what was the problem? Any specifics about actual nightmares would be appreciated.

As a rule, I hardly ever have nightmares, and when I do, it only happens once. I have some recurring dreams that aren’t pleasant, but they aren’t scary, either. I might have them once every two to five years or so. They have to do with bathrooms with water on the floor—perhaps presaging the situation in my work bathroom now. Yucky and weird, but not scary.

I have to wonder if nightmares serve some useful function. Maybe they teach us about things we are scared of? Maybe a certain level of fear is good?

Finally, does anyone know of any science about nightmares and their function?