It is precisely for its having introduced this "ordinary form / extraordinary form" concept that I think Summorum Pontificuм was a "one step forward, five steps backward" landmark in the struggle to reestablish the Traditional Latin Mass.

Msgr. Klaus Gamber, in his book The Reform of the Roman Liturgy: Its Problems and Background, called for an official declaration that the Novus and Vetus Ordos were in fact, two separate and distinct rites. He then suggested that the Traditional Latin Mass be declared the official rite of the Roman Church, and that the Novus Ordo be classified as an "ad Experimentum" rite, which would be permitted to coexist with the TLM for a definite period of time, with the faithful being made to understand that the NO is an inferior rite and that, once the period of "experimentum" had expired, if the NO had not proven itself to have been of spiritual benefit (which, of course, it would not) then it would be declared a failed experiment, abrogated, and the TLM restored as the sole rite of the Roman Church.  

Cardinal Ratzinger's introduction to Msgr. Gamber's book (wherein he famously called the Novus Ordo a "banal, on-the-spot fabrication") seemed to give his full stamp of approval to this plan. Cardinal Ratzinger called the Monsignior "the one scholar who, among the army of pseudo-liturgists, truly represents the liturgical thinking of the center of the Church." One wonders, then, what happened in the mind of Cardinal Ratzinger between the time he wrote that introduction in 1993 and the time he issued Summorum Pontificuм in 2007, wherein he might have implemented Msgr. Gamber's plan, but instead did almost the exact opposite: declaring that the Novus and Vetus Ordos are merely two expressions of the same rite, with the TLM as the inferior "extraordinary form." But then, one wonders many such things about the "pope emeritus."