As I recall, Pathfinder was a test article, much like Enterprise (flight testing article) and where Challenger (structural test article) started... specifically so that the crew of the stacking team had a vehicle of similar weight and shape to practice with until a real shuttle was ready. As such, it's an important part of the fleet, even though it was never meant to fly (unlike Enterprise, which was originally to be converted to space rating, it was ultimately cheaper and easier to finish Challenger's spaceframe).