Dear Des, 

Regnum Christi and other Movements are simply ways to live our Christian vocation. Each group is inspired in the spiritual experience and journey of its Founder, and when the Church approves a Movement it basically says that the spiritual experience and aspirations of the Founder are valid, and the pattern that they set for the members of that Movement is also a valid "spiritual road" for them to follow.  

So Movements have no value in themselves, they have value only insofar as they are part of the Church and are paths that effectively lead their members to draw closer to Christ and live their Christian vocation more fully. Each Movement will have also its own particular characteristic or "charism" which emphasizes some particular aspect of Christian life that its members strive to practice in a special way. Movements also offer a sense of support and family, and usually are engaged in some form of apostolic or service activity that its members are encouraged to participate in.  

So the value of involvement in a Movement for a seminarian or diocesan priest will be an application of its general value for any Catholic: it will be good for him if it helps him live better his own call to the diocesan priesthood. In other words it will be of value to him if for example it strengthens his ties of respect and obedience to his bishop, if it helps him grow in prayer, if it gives him tools for his ongoing formation, provides him with spiritual direction and fraternal support, gives him a sense of the universality of the Church, and if it helps him serve all his people better. 

Take care, and be sure of my prayers.