In my experience I have observed that in general engineers of all disciplines are lazy (some might use the term efficient). That is they never add or change anything without a compelling reason.
Further. Car makers when designing a car deal in 10's to 100's of thousands of units. Any cost savings realized are multiplied by some large numbers.
Bottom line. IF Ford adds something to a part/subsystem used on one application but not on others likely there's a REASON. It may not always be the best reason. Or an obvious reason.
The weight is there to help reduce wheel hop by keeping the front pinion from rotating up during high torque loads. By keeping the pinion down this also improves the performance of the U-joints (angle of the drive line).
Let's think about this. Why would Ford pay extra to add this part to a Mustang but not the other cars that use the same 8.8 rear end? Could it be that a greater percentage of Mustang owners are more likely to drive aggressively? Or could it be that the Mustang has more power compared to the weight of the car and thus a greater chance of inducing wheel hop?
Is it possible that the "dog bone" is a cheaper solution than other live axle methods to reduce wheel hop (think
Panhard bar)? Count on it.
Is it fair to conclude that a part is totally "useless" because nothing bad happens when it's removed? What about the owner that removes the part but never drives aggressively? That owner would likely never see a negative effect from removing the part.
In the same vein. What about the owner that "soups up" their ride and still has problems with wheel hop? Can we conclude that the weight is useless because it doesn't prevent wheel hop? Remember that Ford designed the part be an engineering compromise of cost vs function.
Final thought. Is the part required? No. Will anything bad happen IF it's removed? Depends on how you drive. Wheel hop can be a bad thing. It can damage parts. So you choose based upon HOW you intend to use the vehicle and your repair budget.