Ok, a few notes. I installed both ShAke and defrag. Using both brought my usage up to 5.8% - until I cleared my .thumbnails, Firefox, Flash and gtfp cache, at which point it dropped back to 3.7% - Doing defrag on my home directory reduced this to 2.5% at which it stands now. Note, the biggest issue was caused by cache files.
Secondly, I asked my friend that's running Debian to do a quick check. He has 0.8% for the root file system partition of Debian that he installed 3 months ago. It's minimal and was built from the Debian command line up to use Fluxbox, tint2 and the basic gtk functionality. His home partition however is at 2.8%, both with active usage the last 3 months. That brings the total for the system around 3.2%. Going on your above description you have your home folder with your Slitaz install, while with the initial Debian, it was only the root file system that was counted.
That said, I think your overreacting about it a bit. Even Windows only allows defragging when more than 10% has been fragmented by default and I can almost guarantee it will drop the moment you clear the caches of your applications. Note, I'm at 2.5% now after over a year of active usage. Imagine how bad it would have been for a Windows system by now...