The use of color (or lack) can help emphasize certain points or moods. I think it's best to use a combination of both. Grey scale is good for flashbacks or darker moods, while colour can be used selectively ( Shindler's List) or to indicate some shift in the mood.

Pure grey scale works for some films, but I would bet that most filmmakers who filmed in black and white would have preferred to have colour to depict the vision more clearly. Without careful consideration it's basically just a technical limitation.

That being said, as you mention, the grey method can reduce campiness. Some films that rely on color can lack in other areas.

So, color/grey can be seen as a tool, rather than just a medium.