To some of you guys I'm relatively new to motion, but I have over ten years working with digital files in still photography. While you can shoot both (still and motion) without the use of a gray card, the time saved, with the color neutrality dialed in, in an instant is not worth the fuss if you don't shoot one. It's way to easy to take time to do this. Now it's not the normal workflow for film, but this is digital, and digital is different. I can have your clips going to post without color cast, all day long. Every editor I've worked with only has praises and smiles on their faces. Now I'm not talking about making a color look decision, just getting to "0". Color neutral, meaning skins tones look right, the scene looks right, now where you go from there is your creative decision, your touch, but this time, you start from what looks normal.
Gray Card, absolutely, its a digital photographers best friend. Every professional (advertising, fashion, catalog, editorial) digital shoot done by professional capture firms use this daily, if not, they would spend more time, and money in post. This has been the norm from day one in digital still. RED is a digital still camera, that shoots motion.