i don't see any of the normal problems of a fragged HD with my xbox HD, like slow load and seek speeds, the only part of the HD that is actively re-written is on a seperate partition and therefore doen't affect the load/seek times for any of the games or media.
When you run a normal computer and do anything with it, something gets re-written on your HD. On my computer at home I have all my downloads go to a seperate partition, as well as my windows installation and I have virtually no problems with Defragmenting. I move a lot of data in the downloads partition too, (10-15gig's down, and 5-10 up, yes i'm a leach, but cable has crap for upload), I only have to defrag that maybe once a month( usually %5-10 frag). I'm pretty sure you don't move anywhere near that kind of Data on your xbox. That's why I say the community doesn't need a defrag tool.
Also another problem would be that in testing out a defrag tool any sort of bug in the prog, and you have to know there will be one, that bug would more then likely corrupt your entire HD.
I say the risks out weigh the benifits.