Here you can see a map of wilderness areas overlayed with maps of Inventoried roadless areas. As you can see, they surround formal wilderness areas, expanding them greatly outside of just the formally protected zones, and effectively nearly double the total acreage of wild.

Obviously we all know the implications of clear cut logging and strip mining, which are the main economic incentives to rescind the roadless rule.
But along with the obvious habitat destruction of these activities, the simple existence of roads themselves in a natural environment disrupt habitats, segment wilderness, allow invasive species to spread more easily, disrupt watershed patterns and destroy the integrity of the soil, causing landslides.

The mere existence of a road disqualifies an area to be considered as wilderness.