Two bags maximum or whatever fits in a backpack. I'd like to set something up with a rack just because I think it would be cool to haul something a tad bigger at times.
I'm car-light, and other family members do most of the shopping - by car, but I do sometimes shop by bike, as well as three season bike commuting. I greatly prefer a rack to a backpack as I find it much less tiring and sweaty, especially for biking to work where I try to coast in gently, even on the warmest days, to avoid needing to shower at work. I think most utility cyclists would likely benefit from having a rack. However some younger or more sporty types who commute on the same bike they use for long rec rides or even competition, don't like how the loaded bike handles, or see a rack as spoiling the bike's esthetics or adding weight, and they prefer a backpack for those reasons.