I have only starting using graduated ND's recently, but I have found them to be a great boon in situations where there is a "hot" sky and the horizon line is relatively flat. They've permitted me taking some images that would have been duds otherwise. I'm in Florida, so I find that situation rather easily! If I were in a mountainous terrain, I could easily see where the filters might not be so useful.

Another consideration is that taking two images (one exposed for the shadows and another for the highlights) and merging them in Photoshop is a great alternative to ND filters, making the control of dynamic range completely general and not subject to such vagaries as the shape of the horizon line. It's also easier and quicker to apply in the field, though it does require more film. And some people just like to have the image on the film be the final one ...

Joffre