Chav has become such a widely-used word because it filled a gap: until it caught on, the British middle classes had no conveniently crisp word which condensed all their anxieties about the uppity poor.

As for taste and the Beckhams, they do not have 'no taste', as that isn't possible, but they don't conform to bourgeois parameters of taste.

And I think the key undercurrent of all the chat about chav is OUTRAGE: because chavs are working-class folk with no desire at all to conform to the norms of middle-class codes of behaviour/respectability - how disGRACEful of them!!!