Quite right - 'Weib' and particularly 'Weiber' has a derogatory meaning in today's German Umgangssprache, but as language is a multi-layered thing to us all most Germans (if they listen at all) will gather that Brecht is using deliberately archaic language and that 'Weib' in this song/poem means 'wife' and nothing else, without any derogatory connotations. (Considering Brecht's bohemian lifestyle, it might also stand for 'Lebensabschnittsgefährtin', of course ... :-))