At the most basic level a radiograph is a diagnostic tool. However a radiograph does not possess any intrinsic diagnostic knowledge. It only has value because we have learned to interpret features we see on the image to mean the tissue is either healthy or there is a pathologic change. These features can indicate dental caries, bone loss, periapical pathology or a host of other concerns. We generally make these determinations based on the density of the tissues as indicated by the shade of the image as it varies from black to white and as it compares to similar features on the image.
This rather convoluted description is intended to demonstrate that there is nothing special about a radiographic film, it is the data contained on the film and our ability to see and interpret that data that leads to diagnosis. To evaluate a digital radiograph on the basis of how similar it is to film is to miss the point.
A better evaluation would be to ask does this image contain information or data that I can use to make a diagnosis and can I accurately interpret that data? Based on that criteria a digital radiograph is (or at least has the potential to be) a much better diagnostic tool than a film radiograph. The reason is that a high resolution digital radiograph will capture more individual data points than film. In fact it will capture more data than we can see. The use of enhancement software will allow us to see differences in the data that we can not see with just our eyes. In other words the software can detect differences in the shades from black to white that we can not detect with our eyes.