"But I once wrote, in a Defence Of Categories piece for Folk Review, "If every article of household furniture was called a chair, we wouldn't know where to park our arses." "

Categorisation is fine as long as the descriptors are sufficiently precise. When I lived in South Africa I asked a colleague to teach me some words in a Bantu language (don't ask me which one). We started with colours and he said that there was no word for green - it was a variation of blue - as in 'blue like the grass' as opposed to 'blue like the sky'. I sometimes think we have the same problem with 'folk'.