In return, rather than praising John's action, Jesus instead rebuked him. He reminded John of the value in cooperating with those who work together in Jesus' name instead of dividing people based on perceived proximity to Jesus.
The most interesting part of this story is where it occurs in the narrative in Mark (9:30-50). It seems to interrupt a teaching Jesus gave about discipleship--specifically, learning how to become childlike in your faith and serve others, not stand over them. The wider context is discipleship--what it means to follow Jesus, who, as God's anointed one, was going to be killed (but raised after three days).
The implication is this: following Jesus means emulating him. He was the master servant. He calls us to follow him by serving. And he places no limit on serving: "Anyone who wants to be first must be the very last, and the servant of all" (9:35).
The mission is simple, but the distractions are easy. Jesus calls us into a lifestyle of discipleship by following him as a servant. But John illustrates how easy it is for us to become distracted. It may be the things of life, our jobs, our families, our hobbies.
It may even be our religious thoughts. John thought he was doing something religiously good, keeping Jesus "pure" from those who were not part of "the group." But Jesus drew their focus back to discipleship, to serving, and to him.
Distractions will drag us down. Stay focused on Jesus, growing as a disciple, serving others.