Floyd was smart. He knew Manny was too dangerous five years earlier when Manny beat Oscar and Floyd refused to fight him. Years later when he seen that Manny was slowing down, he reconsidered. It took Manny getting stopped by Marquez to make Floyd realize it was time. He did the same thing with Cotto.
One of the reasons many of us do not give him the same credit as if he fought them when there was too much risk.
Both were bad, Mayweather because he made sure he was fighting sure wins and shouldn't have needed it but Margarito could have ruined a fighter's career with plaster or even have seriously hurt them.