People here know I'm against a deal between the SSPX and Rome, but I think it's important that we list some key reasons as to why they should not sign. In no particular order, here are my reasons. Others are free to add their list their own reasons.
1- The other three Bishops (+Williamson, +Tissier, and +Galarreta) appear to be opposed to a deal. This could mean that, if Bishop Fellay goes ahead and signs a deal with Rome, there could be a major split in the SSPX.
2- The Society would be under the watch and control of Rome. Like the Fraternity of Saint Peter, practically every move they make will be monitored by Rome. This means that their ability to speak out against Vatican II, the Novus Ordo, and any other actions modernist Rome is doing will be very limited, if not completely diminished.
3- Rome has not converted yet. They still do not favor Traditional Catholicism and still adhere to many errors and heresies. The SSPX should wait for Rome to convert first. This was the mindset of Archbishop LeFebvre. The Society never broke from Tradition, it was Rome that did so.
4- Someone even more modern than Benedict could be elected in the future and could attempt to abolish Tradition. Then the Society, if they wanted to do the right thing, would be forced to break away again, making the point of reconciling with them a waste of time and effort to begin with.
5- Archbishop LeFebvre would not have supported a deal, as his own quote seems to indicate:
TWO YEARS AFTER THE CONSECRATIONS:
WE MUST NOT WAVER, WE MAY NOT COMPROMISE
Archbishop Lefebvre's address to his priests given in Econe, Switzerland on September 6, 1990
...A FALSE CHARITY
And we must not waver for one moment either in not being with those who are in the process of betraying us. Some people are always admiring the grass in the neighbor's field. Instead of looking to their friends, to the Church's defenders, to those fighting on the battlefield, they look to our enemies on the other side. "After all, we must be charitable, we must be kind, we must not be divisive, after all, they are celebrating the Tridentine Mass, they are not as bad as everyone says" —but THEY ARE BETRAYING US —betraying us! They are shaking hands with the Church's destroyers. They are shaking hands with people holding modernist and liberal ideas condemned by the Church. So they are doing the devil's work.
Thus those who were with us and were working with us for the rights of Our Lord, for the salvation of souls, are now saying, "So long as they grant us the old Mass, we can shake hands with Rome, no problem." But we are seeing how it works out. They are in an impossible situation. Impossible. One cannot both shake hands with modernists and keep following Tradition. Not possible. Not possible. Now, stay in touch with them to bring them back, to convert them to Tradition, yes, if you like, that's the right kind of ecuмenism! But give the impression that after all one almost regrets any break, that one likes talking to them? No way! These are people who call us corpse-like Traditionalists, they are saying that we are as rigid as corpses, ours is not a living Tradition, we are glum-faced, ours is a glum Tradition! Unbelievable! Unimaginable! What kind of relations can you have with people like that?
This is what causes us a problem with certain layfolk, who are very nice, very good people, all for the Society, who accepted the Consecrations, but who have a kind of deep-down regret that they are no longer with the people they used to be with, people who did not accept the Consecrations and who are now against us. "It's a pity we are divided", they say, "why not meet up with them? Let's go and have a drink together, reach out a hand to them" —that's a betrayal! Those saying this give the impression that at the drop of a hat they would cross over and join those who left us. They must make up their minds.
WE CANNOT COMPROMISE
That is what killed Christendom, in all of Europe, not just the Church in France, but the Church in Germany, in Switzerland —that is what enabled the Revolution to get established. It was the Liberals, it was those who reached out a hand to people who did not share their Catholic principles. We must make up our minds if we too want to collaborate in the destruction of the Church and in the ruin of the Social Kingship of Christ the King, or are we resolved to continue working for the Kingship of Our Lord Jesus Christ? All those who wish to join us, and work with us, Deo Gratias, we welcome them, wherever they come from, that's not a problem, but let them come with us, let them not say they are going a different way in order to keep company with the liberals that left us and in order to work with them. Not possible.
6- It wouldn't be God's Will at this point (most important reason).
Feel free to provide your own thoughts.