But while this consensus was forming the church engaged in other practices that Greg and you reject: penance, confession, indulgences, Real Presence of the Eucharist, and a rudimentary understanding of purgatory. In order to reject this, you have to say that "consensus" only counts for canonicity but not for the liturgical practices in which the Scriptures were used and from which the consensus you cite is extracted.
In addition, why should any Christian accept any Church Council? If the answer is because it is consistent with Scripture, then where in Scripture is the list of 27 NT books? It's not there. Remember that the Scriptura of Sola Scriptura applies to the Bible as a whole and not to its separated parts, for if that were the case someone who just believed in the Book of Numbers as inspired would be obeying Sola Scriptura. But we know that can't be right. So, until the whole is fixed, the disparate parts, though inspired, are not the Scriptura of sola Scriptura per se. After all, many Christian communities did not have the entire collection for generations, though virtually all of them had wholes or parts of what would eventually become the canon. There was no printing press and many Christians were illiterate. And yet, many of them maintained a largely orthodox theology. Why? Because the church universal protected and applied the faith delivered to them by their predecessors. It was a faith inexorably connected to the church's spiritual practices.
But the fixing of the canon itself--the judgment that this is the correct collection of texts--is something above and beyond the text, just as my judgment that defendant X is guilty is not identical to X's guilt. It is a different sort of truth about the defendant. So, sola Scriptura is a theory about the nature of a particular collection of books that we think of as one collection. And yet, it is not something explicitly stated in any of the books. That is, no one book pronounces an explicit and definitive judgment about the canonicity of a collection of books whose authors, for a variety of reasons, did not know about the other books.
So, [F.F.] Bruce is correct that there was a consensus about the canon's contents. But there were also close calls--I Peter, e.g. A church council made a judgment that we today accept. But if all you have is a consensus argument, you have an external standard--consensus--that is adequate to recognize the content of the canon. But then you must believe that consensus is an infallible principle by which one can discover canons. But this results in two problems. 1. It means you have to take seriously the practices I mentioned above--confession, penance--that were accepted widely in the Church. 2. It means that there is an infallible church standard--consensus--by which we can determine canonicity. In that case, you've moved closer to Rome.
It sounds like I really need to listen to the interview that sparked this discussion, because other commenters (seemingly both Protestant and Catholic) were giving all involved some rave reviews.
I think Peter Kreeft and Francis Beckwith are driving at largely the same point: in declaring which books were in the Bible, the Catholic Church provided a clear guide where there had previously been at least some confusion over what represented the authentic Apostolic Tradition (even orthodox Christians offered different lists). So to get from twenty seven books to the New Testament, you can't rely only upon the books themselves (since they offer very little guidance on the issue). So if you're going to treat the entire Bible as infallible (and if you're going to say Christians can't just pick and choose which books they want to be in their Bibles), you need Sacred Tradition.
But, you arguably can't rely upon Tradition alone: certainly, there were Christians (eventually a majority) who argued for the 27 books of the New Testament, but there were respectable minorities as well, and some of the earliest lists we know of are inaccurate. This situation was perhaps more extreme regarding the Old Testament. While all of the early churches accepted the Deuterocanon, the Eastern Orthodox, Ethiopian Orthodox, and various Syriac churches often include more books than even we Catholics, due to later additions to the LXX (these churches don't agree with one another on which extra books should be included, and often, the canon isn't considered dogmatically defined). So even once you get past the "Deuterocanon: yes or no?" question, you're left with "which Deuterocanon?" and lots of different possible answers.
This is a classic case where the Church needs to step in definitively: to rely upon Scripture and Tradition, you need to know which Scriptures, and which Traditions are trustworthy. There is a need for a Church whose decision can be universally trusted, who can say: "this Book is canonical and not that one," and can say, "this Tradition is sacred, and not that one." Catholics are, so far as I can tell, the only Church even claiming that power for themselves. So if you arrive at this point, mentally, there's only one solution to the problem.
But even if you reject the Magisterium, and try and take Scripture plus Tradition by themselves, you find, like Beckwith mentioned, that you're left with a whole lot of tag-along Catholic doctrines, like the Eucharist, the Oneness of the visible Church, the authority of bishops (and a surprisingly top-down organizational structure, even then), etc. To pick and choose which elements one wants to accept from Tradition isn't really following Tradition at all.