There is a certain value of liturgical purity that is validly argued by those who advocate only dismissing the unbaptized catechumens.
This recognizes that catechumens, lacking Baptism, are not yet joined to Christ sacramentally, and would greatly benefit from the additional spiritual nourishment that the Church can offer at the table of the Word of God, as they prepare to join the community at the Eucharistic table.
The other side of this issue notes that, although baptized, the candidates cannot partake of the Eucharist either, and so would also benefit from deepening their experience of the Sunday readings in this special way.
The RCIA text allows for this discernment of pastoral need, without directly calling for candidates to join the catechumens in the dismissal Rite.
(see RCIA 83 and 406).