Mark Shea throws in his $.02 on the matter:
One of the thorny difficulties of trying to heal schism in the Church is that schismatics aren't always brave and plucky rebels whose noble purity of heart led them to bravely oppose grave evil in a Church that Just Didn't Understand Them.
Sometimes, the schismatics are dim-witted, self-aggrandizing, anti-semitic jerks like Bp. Richard Williamson.
So why would the Pope want reunion with them? Well, because the excommunication was never about Williamson's dim-witted, self-aggrandizing, anti-semitic jerkness, but about the consecration of bishops in defiance of communion with the Catholic Church. Likewise, the goal of reunion is not to bless the jerkness of Williamson, but to make it possible for people in need of salvation to return to union with Christ's saving Church. Naturally, the Andrew Sullivans and Little Green Footballs types of the world are, for the sake of various agendas, painting this as "the Nazi Pope returns to his Hitler Youth roots", but it's nothing of the kind. Indeed, no small part of Williamson's agenda is to hold onto his little fiefdom by embarrassing the Church with his antics and keeping his SSPX followers with him and not with Rome. But the sooner sane Trads can get out of the hothose of baleful influence, the better off they will be.
Some Jews are, naturally, upset by Williamson's loony ravings. But I think the Pope is basically operating on the principle that it is the sick that needeth the physician, not the healthy. He can let the SSPX go on being a hothouse presided over by a nut like Williamson, or he can do what he can to bring them into full communion with the Church and hope that the blood of Christ will wash away the toxins of Jew-hatred that have been building up in the system. I will not be at all surprised to hear a) that Williamson takes a retirement after a short interval or B) that Williamson hives off into schism again, but only manages to take a small nucleus of embittered Jew-hating malcontents with him, while most of the rest of the SSPX people stay in communion with Rome.
And, on a related note, the Society of St. Pius I still remains, uh, just as far from full communion with Rome as ever.