The reason using your own totem and not somebody elses was important was because if someone else knew the exact weight/size/mass etc. they could recreate it in a dream, but if no one knew then you could be sure you weren't in a dream by checking if you still had your totem. That's why it was important not to let anyone touch your totem, if they did they could recreate it and you'd never know if you were in a dream or reality. In Cobb's case he can use Moll's totem because she is dead, Cobb is the only one who knows the exact size and weight except for him.

In the end the only thing that matters about the totem is not its owner but the fact that the person wearing it is the only one who knows its properties. (unless the entire film was shown inside Molls dream and she is somehow actually alive )