"Once within the territory of discretion, the court's over arching objective is a fair result. There are of course two faces to fairness. The order must be fair both to the applicant in need and to the respondent who must pay. In any application under Section 31 the applicant's needs are likely to be the dominant or magnetic factor. But it does not follow that the respondent is inevitably responsible financially for any established needs. He is not an insurer against all hazards nor, when fairness is the measure, is he necessarily liable for needs created by the applicant's financial mismanagement, extravagance or irresponsibility."