This is a terrible mistake in my opinion. Christian Origins isn't just about studying the historical Jesus or the rise of the first Christian Jews or the study of Paul. Christian Origins is about trying to map out how an obscure Jewish messianic apocalyptic movement became a Christian religion by the time of Constantine. The second century is the "moment" when this transformation was underway, when the normative process kicked into high gear.
It is a fallacy, although one tauted around frequently as fact, that the "other" forms of Christianity in the second century were "fringe" groups of Christians. Part of the reason for this characterization is that for years we have called the proto-orthodox tradition "mainstream" while all the other traditions "alternatives." Although better than "orthodox" and "heretical," this is language that still gives us a false impression. It is still language that is the consequence again of our theological heritage, the desire to preserve authentic biblical faith of the churches today. Our tradition is "mainstream"; everyone else's is "alternative." This makes it seem like everyone else is on the "fringe" of Christianity, and that they are small, minor or deviant movements.
I have realized the problems with this language only recently. So in my newest book on the Gospel of Judas (The Thirteenth Apostle), I have shifted the language I use to talk about the second century Christians. I now use the term "Apostolic church(es)" when discussing what we have previously called "proto-orthodox" or "mainstream." This shift in language suggests something much closer to the truth: that the Apostolic Church was one variety of Christianity in the ancient world, and in the second century it was not yet the dominant form.
The literature tells us - both the patristic and the NH - that the "other" forms of Christianity were in no way fringe or minor. The Church Fathers tell us over and over again, how massive the Churches of Marcion and the New Prophecy were, how widespread the Gnostic teachings. How concerned were they? Enough to write volumes and volumes against their teachings. Tertullian alone devotes an entire book to depose Valentinianism; five books to criticize Marcion. Irenaeus' Against Heresies is no small feat arguing against minor forms of Christianity. Etc. In the ancient world, where literacy is low and writing expensive and for restricted purposes, the massive amount of rhetoric written against these people is extremely informative.
The literature produced by the "other" forms of Christianity looks scant only because the members of the Apostolic Church burnt it. But these other Christians were equally prolific in their writing and instruction. We happened to get lucky with the NH and Tchacos finds, which recovers part of this other literature. From it we can tell that they were very very sophisticated theologically, and were often critical of theologies of the Apostolic Church. And we can see theologies develop within the Apostolic Church that respond to the criticisms of the other Christians. The theology of the Apostolic Church would not have become what it did without the Gnostics and other Christians (and Jews) as dialogue partners.
The next time we want to dismiss the Gnostic material in particular as late and irrelevant, just remember that a version of the Apocryphon of John existed by the time that the Pastoral letters were written (about 130 CE)! Basilides was our first known commentator on any NT books, teaching and writing around 120 CE. By 120 CE, Valentinus had already set up his school in Alexandria and was a well known theologian. Carpocrates similarly was fully operational at this early date. Marcion (who was no Gnostic) was not only functioning in this period, but had successfully established his own churches with the first NT canon in place (Luke and ten of Paul's letters).
Doug has an interesting post on his blog about this very issue, and how it is perceived by people outside the academic sphere.