It shows a history of consistent performance at the highest levels. Whether they can carry those skills and so forth to another weight class with the same results is a valid question, but it shows that they at least have those skills & attributes in the first place. Aldo, Edgar, GSP, Silva, and Jones are all proven fighters at the highest levels. Rashad is not. That is the difference.
Rashad isn't proven at the top level? He is one of the best fighters in the UFC not holding a belt. Maybe the best fighter not holding a belt.