The real reason to get a dry container is that grinding very hard items like grains will pit and scratch the inside of the pitcher near the blades. In addition to the aesthetic “cloudy” marring of the container, the scratches make the container more likely to hold smells. You can get rid of lingering odors by running the machine with vinegar or a few drops of bleach in water, but it’s nice to not have to worry about that.
Having a second container is also convenient because to grind grains the container needs to be totally dry, and if you made a smoothie in the morning the container might still be wet. Sometimes you can end up using both containers for a single recipe, such as . If you are going to get a second container, it might as well be one with blades optimized for dry ingredients.
For these reasons the dry container is worthwhile, as long as you plan on grinding dry things like grain.