One area that I am interested in is how they help us with interpreting the New Testament. Moises Silva discusses the pros and cons in his study on Galatians particularly the example of John Chrysostom. You can't automatically assume that because he was a native Greek speaker he read Paul 'fluently'. Silva lists three things that should give us pause:
i. Chrysostom is writing 300 years later and a number of semantic shifts in the language would have ocurred in that time
ii. Chrysostom seems to assume that Paul is writing in the sophisticated 'Attic' style of Greek rather than the down to earth Koine style that he actually wrote. This may have influenced his reading of Paul.
iii. This is Silva's most important point: Being a native speaker does not mean that one can read a text and not be left with any ambiguity. Thus, it is a good principal that where Chrysostom (or another Father) recognises a debate over the meaning of an expression in Paul, his views need to be weighed carefully. However, when he comments on an expression without mentioning a debate - in other words just assumes the meaning, this would give us a strong reason for going with Chrysostom's understanding of the expression.
Interestingly, Silva points out the relevance of this for one of the most contentious debates in NT studies - the meaning of pistis christou. This phrase appears in a number of important texts in the NT e.g. Galatians 2:16 and the debate is whether it should be read as 'faith in Christ' (i.e. something we 'do' - reading it as an objective genitive) or 'the faithfulness/faith of Christ' (something that Christ 'does' - a subjective genitive). Silva notes that Chrysostom simply assumes that it means 'faith in Christ'. He argues that this is very strong evidence for this view - that a native speaker simply assumes this reading i.e. he does not even flag that there might be some ambiguity.