The article we're discussing answers this question - NAN gives you backwards compatibility with old versions of V8 (and therefore Node), while the V8 API doesn't.
This is bolded in the article we're discussing:
> NAN's promise is to provide a single API to develop against, not a forever-stable API, it was always anticipated that it would change and adapt to V8 but do so in a way that you get to remain compatible with older versions of V8.