When you are recording your own video clips with a video camera or other device, it is easy to overshoot. Video clips take up a huge amount of disk space.
Movie files are linked to the PowerPoint file, rather than embedded, so they do not dramatically increase the size of the PowerPoint file itself. However, since the linked movie file is required when you show the presentation, having a movie does greatly increase the amount of disk space required for storing the whole presentation package.
Depending on the amount of space available on your computer's hard disk, and whether you need to transfer your PowerPoint file to another PC, you may want to keep the number of seconds of recorded video to a minimum to ensure that the file size stays manageable. On the other hand, if you have a powerful computer with plenty of hard disk space and lots of cool video clips to show, go for it!