From personal experience I can say that they have helped me. I am a better person today than a few years ago with the help of certain books that I have read. I prefer to use the term "personal development" rather than "self help" (although for the most part they are probably the same), because they help you develop into a better person.
But I can see your point about the harmful potential of some of the new-age oriented books as they focus on affirmations, visualizations, and positive energy. There's nothing wrong with those. In fact, they are helpful tools. But a responsible book/author should also emphasize the importance of taking action for individuals who want to achieve certain goals.
It is easier and more pleasant to visualize and meditate, etc. than to take action but if that's all you so, you won't get very far. So the key is to make whatever action you take an inspired action.