I think the goods outweigh the bads here! The absence of plastic bags will force consumers to think about other options - most notably, bringing their own shopping bag along. I always do, and have surely saved thousands of bags - think of that multiplied by a whole city.

Lots of societies don't automatically include a plastic bag for your purchases. In some it's expected that you'll bring your own tote. In Korea, for example, bags are not automatically given - you have to PAY an extra five or ten cents for one.