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's a bad match up for Anderson, anybody not wearing dickhead goggles knows this. You don't like Rashad, we get it. But to say he hasn't proven himself at highest levels is absurd. He was the undefeated LHW champion, only Machida and JBJ can say that too. He has been beating fighters in the top 10 and even top 5 for at least 6 years now, with only 2 loses, two championship fights in which he had horrible gameplans for. So what if he didn't defend the belt? Neither did Machida and Shogun. He still became a champion. I don't buy into all this 'your not the champ until you defend the belt' bullshit that Matt Hughes came up with. Perhaps they aren't proven high level fighters either? He's the only one to go 5 rounds with Jones and actually make it competitive at some points in the fight.
Rashad is just as proven as your Edgar's, your Aldo's and your Silva's.