Solid Neutral Density (ND) Filters
Solid neutral density filters reduce the amount of light entering your lens. The primary purpose of these filters, which are rated by how many f-stops (EV) of light they block, is to allow photographers to use slow shutter speeds in bright conditions. Solid-ND filters are thus useful to anyone looking to use a slow shutter speed for creative or technical reasons. For example, you might want to shoot a portrait using a wide aperture, but in bright light your camera might not be able to use a fast enough shutter speed to properly expose the scene (most shutters top out at around 1/4000 to 1/8000s).
Solid-ND filters are useful for videographers, who often try to capture video with a shutter speed of 1/30s to minimize artifacts. My favorite use for solid-ND filters is to capture extremely long exposures to blur motion and smooth water, for a creative effect you just can’t get normally in bright conditions.
The downside of solid-ND filters is that poorly constructed ones will be extremely prone to color casts and can reduce image sharpness. I use Singh-Ray solid ND filters as they are some of the highest quality glass filters available.
Verdict: If you enjoy shooting with wide apertures or want to capture creative long exposure images, solid-ND filters are a must-have.
About your filters article, I concur with practically all your findings. In early days of digital, some of those filters were really helpful, but now not much. Camera technology is going at super speed. Am curious about where photography is going.