I thought there was a language/linguistics section, but can't find it now so Pedants' Corner is the nearest I can get. Anyway, my point is - I came across this in Middlemarch: "At that time the opinion existed that it was beneath a gentleman to write legibly, or with a hand in the least suitable to a clerk". It reminded me of something I once heard about a tribe in Africa where the men took pride in speaking obscurely.
George Eliot herself gives the likely key, which is to write so that only selected others will understand (ie they have a good idea in advance of what the message is). Does anyone have any similar examples? Or other explanations?