Jesus did not mind that the man was casting out demons by invoking His name. Since the man had such respect for Jesus he would not speak against Him soon. By casting out demons he showed that he was not against Jesus. Jesus expressed the opposite truth in Matthew 12:30 : "He who is not with me is against me." There is no neutral ground regarding one"s orientation to Jesus. Jesus" point was that the disciples should not view the exorcist as an antagonist just because he was not part of their group. He was doing God"s will and would not oppose them. [source][source][source]