I think perpetual engagements are a wonderful means to ensure whole-hearted commitment to one's would-be Fathers in the Priesthood. I would totally be ready to make a perpetual engagement if the time comes. +ABL mentioned once "their Father in Priesthood is me" about Priests who left. I believe there had been cases where people left soon after receiving Ordination from H.G.
And that link of Father Peter Scott that was mentioned earlier - well, I'm not sure what's the point, as Father speaks quite strongly against those who had been Priests in the SSPX rejecting their Superiors (I think this was in Pope John Paul II's time) and going independent. And, if anything, Fr. Peter was very severe and strict, suggesting they may have committed serious sin.
https://m.facebook.com/SspxAgainstTheRumors/posts/1804672902889871 It's probably because of bitter experiences like that they want to ensure fidelity earlier. Others may disagree, but I have no issue with professing not to run away after receiving Ordination.
These two explanations are sufficiently clear: "This event marked the implementation of a new policy for the Society, requiring that any candidate for major orders be perpetually engaged within her family. The desire of the Church that her clergy be firmly planted in one of her dioceses or religious families flows from the doctrine of the Mystical Body. The more completely we are incorporated into the Mystical Body of Christ, the more fully we participate in its life of grace. Of course, we are all incorporated at baptism, and therefore share in the life of Christ, but certainly the Church, a real society, is made up of different families, dioceses and religious congregations, which provide a place and function for each soul. Entrance into one of these families places the soul in closer contact with the life-force of the Mystical Body, realizing its dependence on the same, and directing it with the soul of the Church. For this reason the Society of St. Pius X demands that every soul she gives to the priesthood be submitted to authority, bound for life to her family and in turn bound to the One, Holy, Catholic, and Apostolic Church. Stability and integrity will be the fruits of this commitment, so needed today. Indeed, it is only by submission to authority in God’s order that we participate in His Charity, which is the true bond of perfection."
And from Rev. Fr. Yves Le Roux' letter: "While we have been planting trees here on the property, we have also seen a much more important “planting”: four of our seminarians have just made their perpetual engagements in the Society of St. Pius X. Last weekend, these candidates for the sub-diaconate confirmed for life their attachment to the SSPX, following the new norms established at the last General Chapter. They are thus firmly planted in Holy Mother Church, through the Society. It is indeed essential for the priest to be rooted in the Church, whether through his diocese or through a religious congregation; otherwise, he would be trying to stand alone without support. Without these roots, he cannot withstand the tempest which we know to be very strong in our times. Like Our Lady and St. John, the priest stands at the foot of the cross where he receives the grace to give new life to a decaying society. We can thus be reminded of this grace as we contemplate today Our Lady’s Sorrows in the Stabat Mater. So, let us pray in thanksgiving for our priests, engaged members, and especially for our Sisters who honor their patroness on this feast day ... In the past few months, our mission has not been limited to our own community and associated chapels. The seminary priests have also made numerous trips to SSPX schools and chapels to encourage the young men to seriously consider the priesthood, and to speak to their families about fostering the generosity so necessary for the priestly vocation. Indeed, the “harvest is great, but the laborers are few.” Nevertheless, we are encouraged by the vast field of so many Catholic families, and we trust that God will send forth from them many more workers for His harvest. The “Our Lady of Good Success” chapel in Dickinson, ND is a prime example of this faithfulness and growth of Catholic families ... Without God’s Providence, working through your generosity, this work would be an impossibility. Be assured of our remembrance of you and all of our benefactors in our evening prayers. Our doors are always open for you to visit, and we hope that many will be able to join us for the priestly and diaconate ordinations set for June 21, 2019. While you are here, be sure to stroll the grounds and enjoy the natural and enhanced beauty which are enabled by your prayers and sacrifices. With my own heartfelt thanks and the thanks of this entire community, I wish you every blessing made possible by the Crucifixion, Death, and Resurrection of Our Lord Jesus Christ."
SSPX detractors should remember the only essential things: vocations, holiness, Catholic families, the spirit of sacrifice etc. Not finding fault with each and every decision that informed Traditional Catholic Bishops and Priests have decided to make for pursuing those ends.