The decision to travel via the dirt road or XC (saving half the distance, but certainly not half the time) will depend on local knowledge of the terrain and/or by looking at the detail of the available map in conjuction with the weather conditions predicted. Last thing you want to do is to come across an impassable obstacle such as a cliff face or river, which requires such a wide detour that it makes the other choice of the dirt road much more sensible in the first place.

Trekking unknown XC without a map which doesn't have terrain features detailed such as contour heights and detailed features such as rivers, dense woodland (dense woodland is sometimes easier to detour around) and streams, is not really recommended.

As you can see above from the Jocks Road example it could potentially catch some folks out, if on the map it didn't indicate the type of terrain it passes over.