Now I have been using this patch a long time and , and it really works fine. It applies the DP filter against polygons with a different agressiveness than for lines. This is really useful because strong DP filter against lines looks really bad, while for polygons the visual quality is degraded less. 

I currently use --reduce-point-density=4 and -reduce-point-density-polygon=8
This is perfect for older GPS like etrex to have speed improvements but no drop in visual quality. Also in Mapsource / Qlandkarte GT this works very well. For Oregon or Dakota I would use something like 2.6 (default current behavior) and 5.2


Archived Discussion can be found here: http://www.mail-archive.com/mkgmap-dev@lists.mkgmap.org.uk/msg04260.html
Patch by Johan Gail, working very well since then. (well only that it's not yet set in the help file, so I have to use --x in front of the option)

I added a part2 -- to add the option to the help file, and allow it to be used without --x on the mkgmap call.