Many people make a big deal out of Mayweather's defensive skills and they are pretty good, but not so good that he can't get hit buy even a slower opponent with less on the body movement boxing skills
I think people make it a big deal because he is yet to leave a fight with a single bruise to his face. There's a reason why he is called "Pretty Boy" and that is because he always leaves the ring "looking pretty", as gay as it may sound.

Mayweather likes to duck and cover a lot, I think it's to conserve energy
I wonder if that is why he is selective with his punches and not just throws them at will.

Fighting Pacquiao, Mayweather has three problems he has to deal with. First he has to deal with that southpaw stance which takes away alot of that reach advantage. Second Pacquiao has very good stamina and will have to deal with a lot of punches. Thirdly Pacquiao's speed. This combination is what will give Mayweather his first loss if he ever gets in the ring with Pacquaio. Even if Pacquiao can't get a clean knockout shot on Mayweather he's going to have more connects, and drive the pace of the fight to get a UD
How about if he just keeps throwing flurries of punches only to have Floyr shoulder-roll them, connect Pac with a straight right, and then slip away from him. That's his usual style.