Incorporating it in the spool reel means that every spool reel has to be modified. Mounting it on the extruder only requires one modification. And the entire process is dry so there is no water/windex/other liquid involved.

rather than mount it at the point of entry why not incorp this design into the spool reel. I don't think itd matter much if you were actively printing where in the "line" a filament cleaner is located as long as it does so before entering the extrude head, that way you can still do other things like coloring your filament with sharpies and such without them being diluted with from the sponge liquid or the liquid getting into the extruder head. BTW are you using just water or something like windex, etc... Very cool idea!

Very cool- prints well and mounts well. At first I didn't think dust buildup was a problem, but this morning after a long build I noticed a significant amount of buildup around the filament where it feeds into the drive block.