I have always believed effect is about wilderness encounter maps - not slowing if char steps on swamp or rough terrain in hills.

you know the message like "swamp bogs down you greatly", "rugged terrain slows you down", etc.

Oh I see.
*EDIT
might be nice to see this as a feature. Players who are conscious of game clock try to minimize wilderness travel, and it would be nice to see a class that specializes a bit more in that area so that the penalty is not as bad.
I also wonder if some stuff is simply buggy regarding wilderness travel, given how woodcraft has no apparent effect on the cost of traveling through forest (at least according to the wiki, not actually checked it).