I use them in dialogue, but only if the person speaking is doing so informally.
I use them much more in the expostion:
"He realized that they didn't know. Hadnt they seen for themselves? Perhaps he shouldnt worry."
"He realized that they did not know. Had not they seen for themselves? Perhaps he should not worry."
See the difference. The second example is very stiff, and almost clumsy. Of course I would never write those examples in a story of mine, but it serves the point of the matter.
Really, it depends on the context of your scene and dialogue, and what you want to convey.