just a query,
What more advantage are there for ND filters apart from just flowing water long exposure shots ?
They are far less important than they used to be with film. A ND filter is purely for dumping light when things are too bright.
When using film, and you had maybe 400 ISO film in the camera, on a bright day and on the beache, It could poss go beyond the scope
of usable settings, so an ND filter was your only option. With digital of course you can just turn the ISO down.
There are possabilitys that you may wish to shoot high ISO on a sunny day to gain deliberate grain in your images etc.