We've had this discussion at great length before - a lot of it comes down to what locals expect. In CT, everything that is dirt/gravel/unpaved is marked as dirt - and none of them have a FC over "local" (i.e. "Street"). Users in CT expect "Avoid Dirt Road' to avoid ALL Dirt Roads (including those which are the only way to get to their destination - I've seen the URs).
In Louisiana, there are many, many unpaved, but well maintained roads that have a higher FC and the locals expect to be routed that way - to them a dirt road is more like a pair of tire tracks that you can still get a car down. In Louisiana many of those roads are typed higher and not classified as dirt.