In the eyes of the judges, the guy in top position is in the dominant position. End of story. If the fighter on top stays busy and inflicts damage then this is justified even more.
The only fighter that comes to mind which is dangerous and succesful off their back is Anderson - he will throw elbows, inflict damage and actually GETS submissios.
Usually if a fight goes to the ground and there is not much action it gets stood up anyways.
Is top position over emphasized? Not if it is done properly. A guy like GSP is probably the best at it.