Karl Barth argues in the Church Dogmatics I/1 that the Bible is not the Word of God, but the Bible becomes the Word of God when it is preached by the Church. Karl Barth's theology of the threefold Word of God maintains this important distinction, because the Word of God is never dormant, inside a printed book sitting upon a shelf collecting dust. Instead the Bible is a witness to the Word of God, and when it is read and preached, then it becomes the Word of God. 

Karl Barth defends his thesis with a series of quotations from Martin Luther, where Luther similarly argues that Jesus and his disciples did not write Christian doctrine, but instead they preached the gospel with their living voices. Barth says "And now Luther has the astonishing continuation" and then quotes Martin Luther analogy of the swaddled Christ as follows:

Martin Luther writes, “Therefore even Christ Himself wrote not down His teaching, as Moses did his, but did it orally and gave no command to write it. The apostles, too, wrote little, and not all of them that.… Even those who did write no more than point us to the ancient Scripture, as the angel pointed the shepherds to the manger and the swaddling clothes. And the star pointed the magi to Bethlehem. Therefore’tis not like the New Testament to write books about Christian doctrine, but without books there should be in all places good, learned, spiritual, diligent preachers who draw the living word out of ancient writ and unceasingly din it into the people as the apostles did. For ere they wrote they first had preached to the people and converted them by the living voice, which was their proper apostolic and New Testament work. This is likewise the right star which sheweth Christ’s birth and the angelic message telling of the swaddling bands and the crib. But man’s need to write books is a great injury and it is a violation of the Spirit that the need hath compelled it and is not the way of the New Testament.…” (W.A., 10, pp. 625–28)

It is common knowledge that Jesus did not write any of the New Testament, however there are some theories of inspiration such as plenary verbal inspiration and biblical inerrancy that make virtually no distinction between the biblical author's quotations of Jesus and what Jesus said himself. The fascinating point that Martin Luther makes (and Karl Barth repeats) is that Jesus' disciples wrote almost none of the New Testament either. It's arguably true that none of the disciples wrote any of the New Testament according to the discoveries of modern theologians. Martin Luther (and Karl Barth too) say this is intentional, but Jesus did not command anyone to write down Christian doctrine, and did not command his disciples to write Christian doctrine, nor did Jesus or the disciples command anyone to write Christian doctrine. Why is this true? Because the gospel is a living word that is heard and received through the preaching of the Church. The gospel cannot collect dust in a book, but the preaching of the gospel does occur when the Church studies the bible and then preaches the Word of God from the biblical witness.

Karl Barth improves upon Martin Luther with Barth's doctrine of the threefold Word of God, that clarifies the difference between the written, preached and revealed forms of Word of God (and how they exist in unity). 

So did Martin Luther (or Karl Barth) believe that the New Testament a necessary evil? No! There is no need to bifurcate Luther's distinction between the preached gospel and the biblical writings, because they are both important. Luther (and Barth) believed the Holy Scriptures naturally came about as a result of challenges to gospel by its enemies, so it became necessary later for Christian doctrine to be recorded in the New Testament. The earliest books of the New Testament did not appear until decades after the resurrection of Jesus, so this delay in writing down the scriptures, further prove that Luther (and Barth) are right that the Word of God is preached by the living voices of the Church, and is not printed in dusty books. 

Karl Barth demonstrates this point with this comment, "New Testament Scripture as an adjunct or interpretation of the Old Testament is regarded as a defensive measure against corruption in the Church" and he provided the following quotation from Martin Luther: 

Martin Luther writes “There finally had to be resort to that, and need was that some sheep should be saved from the wolves: so men began to write, and yet through writing, so far as possible, to bring Christ’s little sheep into Scripture, and they prepared thereby that the sheep should be able to pasture themselves and guard themselves against the wolves when their shepherds would not pasture them or turned wolves.”

Karl Barth provides the following explanation of Luther's quotation:

It need hardly be shown, of course, that in other passages Luther attached supreme importance to the written New Testament, that he did not treat it as a necessary evil, and that it is thus quite wrong to hold him systematically to this distinction. But one may still learn from the distinction how he viewed the relation between Scripture and preaching generally. Both have the same theme and content, and this in such a way that preaching takes it first from Scripture and can thus be no other than scriptural exposition, but also in such a way that it always draws from Scripture in the form of living proclamation and has thus to become God’s Word to us. Allegorizing on Lk. 2:12 at the same period, Luther summed up his view as follows:

“Christ is completely wrapped in Scripture as the body in the swaddling clothes. Preaching is the crib in which he lies and is set, and from it we get food and provender” (Sermon on Lk. 2, 1523, W.A., 12, p. 418, l. 24).

Protestant orthodoxy, which at the peak of its development had no liking for talk about the distinction between the forms of the Word of God and the fluidity of their mutual relations, emphasized the more zealously something which is equally true and instructive in itself, namely, their unity.

We may say that "Jesus and his disciples did not write any of the New Testament" because it wasn't their command to write Christian doctrine. If it is proved that John or Matthew did write some of the New Testament, then we may know that it was not their intention to write Christian doctrine, and they wrote what they did for the secondary purpose of defending the gospel that was first preached in the living voice of the Church.

Source:

Barth, K., Bromiley, G. W., & Torrance, T. F. (2004). Church Dogmatics: The Doctrine of the Word of God, Part 1/1 (Vol. 1, pp. 122–123). London; New York: T&T Clark.