Since the papyri have been mentioned twice already in this discussion of the scope of a Hellenistic Greek Grammar, I thought I would offer a little of my own take on the issue.
The papyri are extremely valuable for documenting language change. They are not as helpful for documenting a particular point in the middle of that change. Their very variability offers great evidence for the shifts that were occurring in pronunciation during the Hellenistic period, for example, but they don’t help in understanding Luke’s discourse preferences.
Their importance, from a linguistic point of view, has been that they have given us a clearer picture of conversational or informal Greek in and around Egypt and have clarified to some extent what we might call business Greek or the language of commerce. These are legitimate concerns for a grammar of Hellenistic Greek, but are not necessarily crucial for understanding the biblical documents.
It is very hard to sort out all of the variation in the papyri, but they are nonetheless fascinating.