In the Name of God the Merciful the Compassion

First of all I acknowledge that you are right the Hadith is weak but the scholars have used it. Second: the Hadith is not a source of Islamic law or jurisprudence, the language of the Hadith is Ethical one since it is not contrary with human nature and pure mind its source weakness will not minded.

Third: what has been narrated from the hadith show that there is no problem of using it, if we have a scholar who is more knowledgeable but do not has that characters could we listen to him and consider him our leader?   

Four: it is in the condition of being Faquikh (jurisprudent) to be fair and the hadith shown that he has to be fair more than leader of group pray as Seyid Hakim said in the Book of Al Mustamsik if I am not wrong, that the fairness of Marj’a is a holiness over the fairness of the leader group pray, that’s why if the things are like this it is reasonable to follow someone who has this character rather than other who has none of this character, one of that is hegemonic power or luxury lover… which are the weakness of the knowledge seekers like envy, jealousy of knowledge. The leader of the believers has said: (whoever say: I am scholar he is an ignorant) what about someone who says I am the most knowledgeable. The fairness is part of religion.

All praises due to Allah the Lord of the Universe