I agree that it wouldn’t be reasonable for the Catholic Church (or any Church) to deny membership to people because they are “deranged”. However, these aren’t random members of the Catholic Church; they claim to be bishops and priests, while engaging in these deranged, theology-related rants. If the Pope felt that these mentally ill people should be allowed to have formal membership in the Church, just as a profoundly retarded person would be welcomed as a member, that would be one thing. But un-excommunicating them without explicitly stripping them of their status as bishops and priests, amounts to a tacit endorsement of them as spiritual leaders. It lends the Pope’s authority to their claims to be qualified to guide the spiritual and sacramental lives of lay members of the Church, thus encouraging more people to follow them.
One of the biggest short-term questions regards the standing of the Swiss-based society's bishops and priests -- specifically, whether in the Vatican's view they remain suspended from their ministry until a more complete agreement is reached.
So it's not certain yet what role these priests (and bishops) will play even though the 4 bishops have been excommunicated. That is, this Fr. Abrahamowicz has no pastoral role as of yet, so to say, "But un-excommunicating them without explicitly stripping them of their status as bishops and priests, amounts to a tacit endorsement of them as spiritual leaders. It lends the Pope’s authority to their claims to be qualified to guide the spiritual and sacramental lives of lay members of the Church, thus encouraging more people to follow them." is not entirely accurate.
The priests lead no congregation or parish. They aren't giving Homilies weekly. They have no "spiritual leader" role. Yet.
I hope and pray that the Holy Father will see to it that the more reactionary elements in the SSPX will be expunged and/or dealt with in an appropriate manner.