Smoother slow movements with a gear head. I'm guessing they're referring to slow pans on wide shots. Using a longer lens doesn't necessarily reveal the uneveness a fluid head can bring. If for example the camera is moving through 180 degrees it's not easy to position the body, arm and hand to do it smoothly on a fluid head and maintain a perfect speed throughout. With a gear head it's simply a matter of rotating the crank wheel at a steady pace. A long lens on a gear head might be difficult to keep something fast moving in shot because your having to spin the wheels back and forth really quickly to keep track of it, where as a fluid head is far more natural at following something further away in a smaller FoV (Imagine chasing a bird in the sky with a fluid head compared to a gear head using a 300mm lens - obviously this is quite an extreme example but an actor moving irratically about a set can generate similar problems). Of course a good op can get good results with a fluid head even on wide lenses which is why gear heads aren't as common, that and the fact that fluid heads are point and shoot, quicker to setup shots and generally lighter.
or to get a smoother camera movement in motion picture...
