Another argument to add:
Many of the existing Wikipedias (Catalan, Basque, Galician, Sicilian, Scots, etc etc) are in languages whose speakers are generally also highly fluent in a more widely-spoken language (Spanish or French, Spanish or French, Spanish or Portuguese, Italian, and English, respectively).
Should we delete these Wikipedias?
I think that, with time, a Neapolitan Wikipedia could grow to be the size that the Italian Wikipedia is today. Will they ever be the same size at the same time? Probably not. But it's also unlikely that the Italian Wikipedia will overtake that English Wikipedia in size... it seems to have to do more with when they were started, how well they were publicised, etc., rather than some sort of internal value for each language.
Mark