Before pontificating on my own account, I note that I take the Pope's "explanatory letter" at face value. He is clearly saying just what he believes; there's no reason to suspect any hidden agenda; and he's clearly consulted widely about this. His motives are pure, his intentions good. And I like having the older rite around, even though I vastly prefer a well-done Novus Ordo Mass in Latin, and once sung regularly in a polyphonic choir for such a liturgy. The Tridentine Rite has helped to keep appreciation for Tradition alive amid the massive loss of culture, and in some cases even of memory, that Catholicism saw in the generation following Vatican II. Even so, I don't think Benedict's new move will or can usher in anything more than a transitional phase.
As he himself indicates, there aren't that many priests out there with the liturgical training and knowledge of Latin necessary to make celebration of the Mass according to the 1962 Missal widespread. For that reason, he seems to believe that bishops worried about this move's sowing disunity in their dioceses are mistaken. But the truth, as I see it, lies somewhere in between.
Despite the best of intentions, this move will legitimate what is already a de facto schism. For the Tridentine Mass is the rallying point for Catholics, clerical or lay, who think Vatican II was unnecessary at best and a disaster at worst. The new indult will make it easier for lay Catholics of that sort to attend Mass in that form; many of them already attend such a Mass but do so despite resistance from their bishops, so that some of the priests who preside at such Masses are doing so illicitly—either without "faculties" or as members of schismatic groups. Greater lay access will do little to change the attitude of such priests toward either their bishops or to the "updated" Church in general; what it will do is make attendance at their liturgies guilt-free; and in the medium term, the most conservative bishops will probably permit seminarians to learn how to do a Tridentine Mass. The effect of all that will be to make it easier and more respectable to be what the traditionalists already are, which is a much wider reality than that of liturgy alone. In effect, it will further entrench a spiritual culture sharply at odds with much of what has happened, good as well as bad, since Vatican II.
That does not mean I oppose the new indult. As a tactical matter, it is probably the only way to bring closer that "reform of the reform" which Joseph Ratzinger has long thought necessary. I suspect that's probably what he now thinks too. For the effect of the Tridentine-Mass culture on the wider Church will be to make it more respectable to seek out liturgy that isn't schlock and is more organically connected to the great past than the Novus Ordo has been. My own preference, for example, has long been to bring back Latin for the Ordinary of Mass and celebration ad orientem by the priest, neither of which Vatican II or the Missal of 1970 forbade, but without sacrificing those aspects of the Novus Ordo I like, such as the wider lectionary cycle in the vernacular, lay lectors, and communion in the hand. My hope and prayer is that easier access to the "extraordinary" form will make that possible in many quarters where it now is not. But in the meantime the price will be high.