The Church makes a pastoral provision for the priestly ordination of former Protestant ministers because it seems prudent to allow those who have served as ministers to continue to pursue the calling to ministry that they feel, that they have trained for, and that they have exercised throughout their careers. The prudence of this pastoral provision is especially obvious in the case of Anglican ministers who desire to become Catholic along with their parishes. In such cases, the minister has a special relationship with his parishioners that facilitates the full communion of the entire group.
However, in other cases it is true that a minister with a family would have difficulty serving as the pastor of a Catholic parish. In these cases they are not assigned as pastors but serve in another capacity.