I have the impression that many people at this time as well as in preceeding times, married people with whom they had not long acquitance or did not know very well. That seem to explain Mr and Mrs Bennett's form of marital address. Correct me if I am wrong, but the newly married Mr and Mrs Collins call each other 'Mr' and 'Mrs' as well... I don't know if authors of the time were constrained to use polite forms of address among the charecters.