The problem isn't people who reuse their carrier bags as bin bags, but the people who get new single use bags every time they go to the supermarket and don't reuse them, but let them build up to epic proportions in their cupboards and then throw them away.
And also the mindless morons that litter and just let them blow around until they get stuck in trees etc.
Hopefully, having to pay for these bags will focus their minds and make them reuse them, or buy other resusable bags.
I am a little annoyed about having to pay for bags (I reuse all mine as kitchen bin bags and carefully manage the amount we use - we have a combination of bags for life and single use bags, so that we have enough to use as bin bags). But it's a tiny cost and fair enough if it cuts down plastic waste.
When the charge comes in, I will look into whether it is cheaper to buy bin bags or buy single use bags to use as bin bags.
But the other issue is quality - UK supermarkets, with the exception of M&S and perhaps Aldi that I have noticed, so they can say they have reduced the amount of plastic used, have made the bags so uselessly thin that you are lucky if you get the bag home in one piece.
This is short sited IMHO. If the bags tear so easily, there is no hope of reusing them. I have a normal plastic single use bag that I bought for 4 cents in a Spanish supermarket in May that is much thicker and has been reused many times in the past 3/4 months - the UK ones taken at the same time would have been in landfill straight away - half of them can't even be used as bin bags.
So it would appear that in England at least, we have taken the stance that there is no hope of making people behave responsibly, by making them reuse their bags, so we will make the single use bags as thin and crappy as possible, so the amount of plastic thrown away is reduced and we can claim that we have done what we are supposed to.