The argument goes something like this: the existence of distinguishable text types testifies to the strength and relative isolation of the individual churches which produced and preserved those manuscripts. This suggests, it is argued, that early Christian literature was produced primarily by and for individual communities.
I confess that this argument strikes me as specious, and I'm wondering if someone can help me see if I'm missing something.
While the list of commonly acknowledged text types may look impressive, it is misleading. The Byzantine text is thought to be, in part, the result of conflation of earlier text types, so it actually testifies to the interpenetration of streams of textual transmission in Christian antiquity, not to their isolation. Similarly, Bruce Metzger described the Caesarean text as being characterized by a mixture of Western and Alexandrian readings and as "the most mixed and the least homogeneous of any of the groups that be classified as distinct text types" (The Text of the New Testament, 2005 p.310-12).
Now we're left with just the Alexandrian and the Western texts, both of which appear to have emerged at some point during the second century, in the case of the the Western text the latter part of the second century. The existence of two identifiable text types does attest to some degree of isolation. Nevertheless, the significance of this relative isolation for the question of Gospel audiences is greatly attenuated by the fact that we're talking about a period of time 50 to 100 years after the composition of the Gospels. And of course both text types attest to all four canonical Gospels.
To sum up: when it comes to the debate over Gospel audiences, it seems to me that only the Alexandrian and Western texts count as evidence of the relative isolation of streams of transmission of Christian literature, and this evidence is too late and too weak to tell us much, if anything, about the concerns of the Evangelists.
I'd love to be shown if I'm missing something. That's what blogs are for, right?