another reason is noise, i know on the dash some captains would keep the #1 in feather because the FA was sitting next to the door and well its loud. also if you just landed most turbine engines need a cool down period so to speak in feather. so why not just do it on the taxi in with one in feather and you can shut it down that much sooner. instead of parking at the gate and waiting for 30 sec to a minute to turn them both off.