I can carry one of those in each arm or maybe three total bags, if I balance it right. With the plastic ones I can get just about the whole trunk-load wrapped around my wrists and up the stairs in one go.
Seriously, when I was a young man in college I would ride back from the grocery store on my bike with about 50 pounds of groceries in double bagged plastic balanced on my handlebars. Life with only paper bags would have been very sad.