If you're looking for a more nuanced and detailed take on the Vatican's decision to lift the excommunication of four Society of Saint Pius X bishops than, say, the New York Times provides, I recommend Amy Welborn's roundup and analysis. This bit, especially, distills what I'm assuming is the essence of the reasoning behind Benedict's decision:
The Pope is not stupid. He knows the ins and outs of the SSPX better than any of us and is deeply familiar with the various currents of belief, practice and attitude that run through it. There are virulent anti-Semites in the SSPX. There are near-sedevacantists. There are many who believe that the Second Vatican Council was an illegitimate, invalid council. There are those who believe that the Mass that most of reading this blog go to every Sunday, if not every day, is invalid and that the elements are not consecrated.
But he also knows, particularly in Europe, there are many SSPX adherents who do not share these views and are simply seeking to practice a richer Catholic faith than is available to them in their local regular parish. I think to really understand the whole picture on this, you have to understand the European situation, which in many ways is quite different than it is here.
I think what the Pope knows is that there is going to be a huge degree of self-selection going on over the next few years, as well as some inevitably self-destructive behavior. In short, those who truly want to be union with Rome will do so, and the holdouts will hold out until some fantasy moment occurs in which the Novus Ordo Mass and the Second Vatican Council is repudiated.
The problem, of course, is that by create this opening for those SSPX-ers who should be in full communion with the Catholic Church, the Vatican is temporarily empowering Bishop Richard Williamson, Holocaust denier and all-around charmer, who gives every evidence that he shouldn't be - and probably doesn't want to be - back in the fold, but who's instantly become the poster boy for the Pope's decision, and for the Traditionalist community more generally. This is a price worth paying, hopefully, for the sake of closing unnecessary divisions, but the price wouldn't be nearly so steep if the Vatican had a better sense of how to do public relations in a controversial case like this. The average reporter or commentator isn't going to understand the nuances of canon law, the history and background of the SSPX, the context of the excommunications, the status of these bishops post-excommunication, and so forth. What the average journalist does understand, though, is how to write this headline: "Pope Rehabilitates Holocaust-Denying Bishop." And while the potential for bad publicity shouldn't prevent the Vatican from showing mercy to excommunicants when appropriate, it should incentivize wrapping any such mercy in a forceful, detailed, "Catholicism and canon law for dummies" explanation of what such an action doesn't mean: In this case, an endorsement of poisonous anti-semitism and conspiracy theorizing.
And this is exactly what hasn't been forthcoming. Oh, the Papal spokesman said that Williamson's Holocaust-denying remarks were "completely indefensible," and L'Osservatore Romano had an editorial (not yet translated into English, of course) stating that the decision "should not be sullied with unacceptable revisionist opinions and attitudes with regard to the Jews." But in the contemporary media environment, that's not good enough. If the Pope de-excommunicates a Holocaust denier, the Vatican press office should be working around the clock, with press releases flying, to provide context and do damage control. What's more, if the Pope de-excommunicates a Holocaust denier, the Pope himself needs to say something about it, and not just obliquely nod to the decision in his latest homily. Yes, the Church's primary business is saving souls, not public relations - but in this day and age, public relations is part of the business of saving souls. And nobody in Rome, from Benedict on down, seems to have figured that out.