You've got it. If you are using a slow film and high shutter speed and still can't get the wide aperture you want, then ND filters are the solution. It's nice to have a set of three or four of them spaced one stop apart for different lighting conditions and effects. You can combine them, but that adds additional surfaces that can attract flare. If you like to do some long exposure effects, you can get an 8 or 10 stop ND filter for that purpose as well.