If you post a marketing video, you want the most possible people to view it, right? So why make it so that only those who have the latest, up-to-the-minute versions of software installed can watch it? When you do that, you lose a lot of potential customers!
Unless your video is demonstrating the latest special effects game, software, or movie, and your audience is young, affluent geekboys, you don’t need the latest version of Flash, released 5 minutes ago.

