I've been away for a while, so I'm not sure if anyone's still watching this, but still. I should say that I disagree with most posters here.
The basic fact you all seem to conveniently miss (for the noble reason of gameplay, I can appreciate that) is that in any case, flavor or no flavor, it is still EASIER to move on a road rather than on plain terrain. The way it is now, it makes no difference for an invading army whether there is a road in the tile! That's just stupid. Again, imagine an attacking general thinking: "Oh, well, it's an enemy road that goes through this mighty forest, so it's of no use to us. We might as well go a mile to the right and proceed through the very thicket, with our supply trains, camp followers and all. Also, our mighty crusaders will jump at any strange sound, so it will hamper their progress on a road. Luckily, they aren't so jumpy in the absence of a road, so their movement costs in the wild terrain remain unchanged. It's nothing that we've just crushed the enemy's main force in the field and his virtually unguarded capital is only 4 tiles away on good imperial road, that his territory is swarming with our assassins and rangers and constantly surveyed by our hawks. We've got to proceed with caution - it's enemy territory, after all. Oh, and have you seen those brave Sheaim and Calabim peasants standing up for their country? Did just as well as Elohim last year, I tell you. At least, the movement cost was the same for us".
Another flavor-wise objection: why do elves, dwarves and the rest retain their terrain or feature movement bonus in the enemy territory? It makes good sense from my point of view, but how do you explain the uselessness of roads then?
Now, about destroying roads. It's basically the same objection I have: DESTROYING many miles of road so as to make it just as unsuitable for travel as the surrounding terrain is a mighty feat indeed. "I see a good stretch of road ahead, up to the horizon, sir! - Wait! What's that blotch about 10 miles down there? Damn! There are fallen trees and a ditch across the road! We might just as well go through the plain country, the road is of no use to us anymore."
So, while I can agree that you probably shouldn't get full speed on enemy roads (for the sake of gameplay, if not realism), you should still get some bonus movement.
As for all this commando business, i think that assassins and spies (yes, spies should be back in. They are in Orbis) fill the niche neatly. You don't really want a horde of special troops, do you?