Take a few minutes to look at the following slideshow. It is a convincing argument against the use of plastic bags, backed by quotes from credible sources.
It is well worth watching.
Web Link

I know Mountain View has banned their use, Palo Alto is talking about charging. Good link.

Feel bad for those animals.

The other side of the coin of this ban is that conscientious users of plastic bags employ them to hold garbage and one or two a week are disposed of in the black cans. If the bags are banned at stores, one then needs to go to the store and buy a package of plastic bags for that purpose. These polyethylene bags come in commercially in various sizes to fit typical containers that hold disposables inside the home.

The problem with the currently used bags is that too many people toss them and misuse them. They are the same people who violate all the littering laws and cause huge costs for cleaning up the roadways and even parks. That litter will continue or get worse if one does not have a plastic bag in the car to hold garbage. More stuff will get tossed by those who do not care and violate the law, potential fines not withstanding.