And here is a good overview on the "these are the oldest thus best" idea:
"The oldest representatives of the "purely" Alexandrian group of texts are the two "great" uncials, Sinaiticus and Vaticanus (Aleph and B, respectively). Both of these manuscripts date to the 4th century, with Vaticanus proffered as from the latter quarter of the century, and Sinaiticus being from around the middle of the century. Several scholars have even suggested that these uncials are two of the original 50 copies of the New Testament text which were made by Eusebius for official Church use at the behest of Emperor Constantine.5 Thus, the oldest pure Alexandrian manuscripts date to around 350 AD and after, nearly three centuries after the penning of the original autographs. The problem for the antiquity interpretation of the modern textual scholars which immediately arises is that corruption (both accidental and purposeful) in the New Testament text was greatest in the first two centuries after the revelation of the New Testament (roughly 80-200 AD). Scrivener argues that the worst corruption to strike the New Testament texts occurred within a century of their composition.6 Further, Colwell states that "The overwhelming majority of readings were created before the year 200..."7 It was during this period, while many books were still in the process of filtering out to Christian communities all across the Empire, that heretical texts would have been easiest to introduce and pass off as legitimate Scripture. Kilpatrick argues that with the advent of the 3rd century, it then became nearly impossible to change the text of the New Testament in a way which would have been either accepted or unnoticed by Christians at large, "Origen's treatment of Matt. 19:19 is significant in two other ways. First he was probably the most influential commentator of the Ancient Church and yet his conjecture at this point seems to have influenced only one manuscript of a local version of the New Testament. The Greek tradition is apparently unaffected by it. From the third century onward even an Origen could not effectively alter that text. "This brings us to the second significant point - his date.
From the early third century onward the freedom to alter the text which had obtained earlier can no longer be practiced. Tatian is the last author to make deliberate changes in the text of whom we have explicit information. Between Tatian and Origen Christian opinion had so changed that it was no longer possible to make changes in the text whether they were harmless or not."8 Thus, even by the 3rd century, and definitely by the fourth, the Scriptures were more thoroughly distributed and Christians were better able to compare texts and reject heretical manuscripts. Once the faithfulness in transmission for the texts had solidified, the issue then becomes one of competing textual lines, between which Christians of that age had to choose. This is where the age of the Alexandrian exemplars actually works to the detriment of modern theories based upon antiquity.
What needs to be understood about the ancient manuscripts is that there were basically two types of media for texts - vellum and papyrus. Neither of these media are especially durable. Vellum (dried skins of sheep or other animals) was more rugged and expensive, and was used in the copies of the Scriptures held for "official" use by the churches, and by more wealthy individuals. Both Sinaiticus and Vaticanus are vellum manuscripts, and as such, were probably intended for use in Christian assemblies or liturgy. However, vellum scrolls will wear out over time through use and need to be replaced (just as a well-used Bible today will tend to do). Back in the day, they did not have rebinding services like we have for Bibles to give added years to the life of a scroll, so the scroll had to be transcribed into a new manuscript....
The obvious point to all this, then, is: "why are such old exemplars even still in existence and in the relatively good condition which they are, since they are over fifteen centuries old?" The answer suggested by numerous scholars such as Van Bruggen, Pickering, and others is that these scrolls are in good condition despite their age because they were never used... "
http://www.verhoevenmarc.be/PDF/GnosticCorruptions.pdf