I do the same. Plastic bags are quite convenient, are often easier to clean than fancy reusable bags, can be given away without a second thought, and fit nicely in a backpack when folded. Whenever I get a new one, I fold it up and put it in a box in my coat closet. Going shopping? Grab a few, stick them in my car or backpack.

They last practically forever—certainly longer than the cheap reusable bags that have a tendency to tear and stain.