Flexi synning stems from the days when you could only have your syns on a daily basis. You had your 5 - 15 syns a day and that was that. But then if you were faced with a special occasion what did you do? So flexi synning was a way of staying in control of what you were eating without feeling guilty about going over your 15 syn limit. Now that lots of people have their syns on a weekly rather than daily basis it perhaps isn't so important because they have the option of saving up syns for going out.