The plastic grocery sack (aka "t-shirt bag") is one of the most fantastically useful gizmos to have been invented by the human race. Thanks to the magic of its loopy handles, I can drape several over each wrist and schlep a week's worth of groceries up the stairs, yet still have a hand free to operate the knob at the top. But getting the groceries through the front door is only the start of their usefulness.
I have one of the bags hanging on the wall near the kitchen, stuffed full of other bags awaiting all their future tasks. For instance, I keep one by my computer desk to use as a receptacle for soft drink cans, kleenex, and suchlike. When it fills, I take it to the main trash receptacle and replace it with a fresh one. There's a double-bagged pair atop the fridge, serving as a cache for things I don't want to put in the regular trash can, lest they wind up serving as cat bait, such as empty chili cans and whatnot. They let me take books to work in the rain without fear of getting them wet, even bungeed on the back of a bike. They serve as containers for the contents of my coin jar on its monthly emptying. They make great impromptu rain hats. They're effectively free, disposable, fantastically wonderful little items; triumphs of human ingenuity. If they made better bookmarks and were edible, they'd be perfect.
For all these reasons and more, some folks just hate 'em. The hair shirt crowd, those who are convinced that anything fun or useful must be unhealthy or cause global warming, have now succeeded in getting them banned in San Francisco. Supposedly you can replace them with a canvas bag (and how often am I going to have that with me when I spontaneously decide to stop at the grocery store on the way home from work? And will it hold a week's worth of groceries?) or recyclable paper (everyone who's ever heard that sickening tearing sound followed by the clank and thud of beer and Ben & Jerry's hitting the linoleum, raise your hand).
If you're going to San Francisco, be sure to smuggle some plastic grocery sacks.