Unless you have a site that is featuring just hi end streaming video, and have a LOT of it, I think you really need a good balance of vids and still photos. As others mentioned, just for all the areas that need stills for promotional and advertising venues: banners, site tours, tgp galleries, fhg's, freesites, full page ads and so on, you need them ... and speaking as a designer here ... it's a real bitch trying to put out quality ad media using vidcaps.