It is important to put the question of defrag into perspective. As I mentioned, and I think everyone agrees, disk fragmentation mostly effects large files, such as large raw video files. If you have a very specific reason to defrag (such as working with large, disk dependent video files, where disk is the bottleneck), and speed is very important then it could help. A 10% performance increase on a job process that takes 10 or 20 hours would be significant. A 10% increase on the time it takes to launch word would amount to less than a second ....
Browser/Plazma, just out of curiosity does your video editing package not store its page/swap file on a single, contiguous disk space, like the windows page file?