Good, it's really not that hard to remember to take a bag of some sort with you when you go out, and if by chance you are caught short then paying for them might make you think more, it's just habit you need to get into really.
Having said that I'm with threadworm on the price of them, is 5p really enough? I mean you're more likely to forget bags when you pop in for a loaf of bread than do your weekly shop so will probably only buy one bag making it not really matter iyswim.