From The Gospel of Mark by Mary Healy, comnenting on Mark 9:39-40:
Jesus’ reply directs his disciples to take an expansive rather than a restrictive approach toward others who are acting in his name: Do not prevent him. There is no place for exclusivism among those who invoke the name of Jesus. Paul illustrates a similar principle in Phil 1:15・18: “Some preach Christ from envy and rivalry. . . . What difference does it make, as long as in every way, whether in pretense or in truth, Christ is being proclaimed? And in that I rejoice.” The disciples should not presume to restrict the invocation of Jesus’ name, because no one who performs a mighty deed in his name can at the same time speak ill of him. There were plenty of people who did speak ill of him, and Jesus will take all the friends he can get! To do a work of healing or deliverance in his name is to honor him, and is not compatible with being his adversary. For whoever is not against us is for us. Jesus is directing the disciples to take a stance of openness toward those who are not within the formal bounds of the Christian community, and not to consider them foes. But the converse is stated in Matt 12:30: “Whoever is not with me is against me.” In the end there is no neutral ground in relation to Jesus: sooner or later everyone chooses (consciously or unconsciously) either to be on his side or to oppose him.