So I have been told that a Variable ND filter is a must for recording video, especially in event situations (like weddings), or anytime your preference is a run-and-gun style of shooting; where you have no control over lighting. And since your shutter speed is usually fixed, your preference for aperture is generally set, leaving really only a small window with ISO - a variable ND filter just makes things so convenient. At least, more convenient than using set ND filters.
After using a variable ND filter, I definitely agree that they are a must have - though they do come with their drawbacks.
Currently I own the Polaroid Optics Variable ND Filter and the LightCraft Workshop Fader ND Mark 2. The former runs around $55, depending on what size, and the latter runs around $129, also depending on what size you get.
Here are some test shots with them to help point out the drawbacks.
Images using a Canon 5DmkIII, 70-200mm f2.8 at the 200mm range
(Click on images to open a lightbox to view better)
Notice the striations in the bokeh in the Polaroid VND image, especially in the yellow marigolds. Also, you can see how the contrast flattens with the VND filter in place, as compared with the original image.
Here you can notice the lessening of detail when a VND is used, though this is more apparent when pushed to maximum zoom in zoom lenses. The plant images have been enlarged to show both the striation drawback and detail loss.
Images using the 24mm f1.4 with the 5DmkIII
However, once again, the LightCraft Workshop filter does a better job than the cheaper Polaroid Optics filter.
PLEASE NOTE that neither of the two VND's are weather-sealed. Meaning small, or fine, dust particles may enter between the two rotating glass elements. So if you are shooting in a desert, or a very dusty place, bear this in mind.
Overall, I still think the VND filter is a necessity for the videomaker, even a set of standard ND filters. It is as simple as having the tools necessary to shoot in various settings.
I'll revisit this when/if I get a more expensive VND filter.
For video comparisons, 90% of my videos currently use the filters. If you think a video comparison would help you out better than just the above image samples, let me know and I'll see what I can put together.