Hi all - This is my first post here, so hope I do it right. I have often thought that a low ISO would be helpful and let you eliminate using ND filters on many occasions. I don't like to use filters unless needed (just more glass in the way).
My current camera is a Nikon D80 with a base ISO of 100. I have often thought it would be nice to have 25, but would not use it unless needed (as the OP said, waterfalls, streams etc). I do use 100 much of the time on my tripod if I'm not worried about freezing action.
I doubt that 25 would give you any gain on the noise level over 100 or 200, unless that was the base ISO of the camera (another point we can debate) - Bruce