The Supreme Pontiff has made it clear that the Missal of 1962 is a valid form of the Latin rite. In fact, it is the extraordinary form of the Latin rite while the Missal of 1970 is the ordinary form of the Latin rite.
At the time of the introduction of the new Missal, it did not seem necessary to issue specific norms for the possible use of the earlier Missal. Probably it was thought that it would be a matter of a few individual cases which would be resolved, case by case, on the local level. Afterwards, however, it soon became apparent that a good number of people remained strongly attached to this usage of the Roman Rite, which had been familiar to them from childhood. This was especially the case in countries where the liturgical movement had provided many people with a notable liturgical formation and a deep, personal familiarity with the earlier Form of the liturgical celebration. We all know that, in the movement led by Archbishop Lefebvre, fidelity to the old Missal became an external mark of identity; the reasons for the break which arose over this, however, were at a deeper level. Many people who clearly accepted the binding character of the Second Vatican Council, and were faithful to the Pope and the Bishops, nonetheless also desired to recover the form of the sacred liturgy that was dear to them. This occurred above all because in many places celebrations were not faithful to the prescriptions of the new Missal, but the latter actually was understood as authorizing or even requiring creativity, which frequently led to deformations of the liturgy which were hard to bear. I am speaking from experience, since I too lived through that period with all its hopes and its confusion. And I have seen how arbitrary deformations of the liturgy caused deep pain to individuals totally rooted in the faith of the Church [emphasis mine].
The two primary fears voiced prior to the publication of Summorum Pontificum, both that the motu proprio would undo the reforms of the Second Vatican Council and that it would cause division in the Church, are "quite unfounded."
The Supreme Pontiff explains that the Missal of 1962 was "never juridically abrogated and, consequently, in principle, was always permitted." As to the second fear, he says:
The use of the old Missal presupposes a certain degree of liturgical formation and some knowledge of the Latin language; neither of these is found very often. Already from these concrete presuppositions, it is clearly seen that the new Missal will certainly remain the ordinary Form of the Roman Rite, not only on account of the juridical norms, but also because of the actual situation of the communities of the faithful.
"There have been exaggerations and at times social aspects unduly linked to the attitude of the faithful attached to the ancient Latin liturgical tradition," His Holiness acknowledged. "Your charity and pastoral prudence will be an incentive and guide for improving these," he wrote to his brother Bishops. Much of the talk in recent weeks has focused on the negative reasons for the issuing of this motu proprio, but as is his customary style, Pope Benedict offers the positive reason for his decision:
It is a matter of coming to an interior reconciliation in the heart of the Church. Looking back over the past, to the divisions which in the course of the centuries have rent the Body of Christ, one continually has the impression that, at critical moments when divisions were coming about, not enough was done by the Church’s leaders to maintain or regain reconciliation and unity. One has the impression that omissions on the part of the Church have had their share of blame for the fact that these divisions were able to harden. This glance at the past imposes an obligation on us today: to make every effort to unable for all those who truly desire unity to remain in that unity or to attain it anew. I think of a sentence in the Second Letter to the Corinthians, where Paul writes: “Our mouth is open to you, Corinthians; our heart is wide. You are not restricted by us, but you are restricted in your own affections. In return … widen your hearts also!” (2 Cor 6:11-13). Paul was certainly speaking in another context, but his exhortation can and must touch us too, precisely on this subject. Let us generously open our hearts and make room for everything that the faith itself allows.
There is no contradiction between the two editions of the Roman Missal. In the history of the liturgy there is growth and progress, but no rupture. What earlier generations held as sacred, remains sacred and great for us too, and it cannot be all of a sudden entirely forbidden or even considered harmful. It behooves all of us to preserve the riches which have developed in the Church’s faith and prayer, and to give them their proper place. Needless to say, in order to experience full communion, the priests of the communities adhering to the former usage cannot, as a matter of principle, exclude celebrating according to the new books. The total exclusion of the new rite would not in fact be consistent with the recognition of its value and holiness.
The entire letter is beautiful and extremely well crafted. You really must give it a read.