Most if not all airlines require multi-engine time because of their insurance requirements. Wether single engine turbine time or mutli-piston time is better is really not the issue. Sure a modern turbine single could statisically be safer that a 25 year old Barron (though you'd never catch me in any IFR single on a dark and dirty night), the point is that flying a twin forces the pilot to always consider the options if a failure occurs. It is this thought process that the airlines are looking for when they require 500 hours on twins. Ask yourself this question,are there any of these intergated airline backed flying schools offering single turbine time in place of the Duchess when they train thier IFR stuudents?