I sometimes carry a small belt knife but I do it discretely. I wear it high on a Tek-Lok and usually in concert with a flannel, jacket, or other cover garment. We've already got some good advice on why but here is mine:
1. In a dangerous situation in which people might have to defend themselves, why tip your hand to the bad guys?
2. There are lots of people who have little to no experience with guns, knives, or even basic utility tools. Many of these people have had their worldview shaped solely by media. That includes negative depictions of guns and knives, both in fiction and on the news. It also includes propaganda that incites fear of such tools and disdain or hostility towards the right to carry them and those who do. While I'd love to see those folks properly educated on the subject, there is no value in making them uncomfortable at the supermarket. In fact, fear and ignorance can be a dangerous combination. Since safety is the goal of carrying an emergency defensive tool, it makes sense to keep it concealed.