Limits a post to a single category by changing the checkboxes into radio buttons. Simple.
Only One Category makes sure users only select one category for their post by making the selectors radio buttons, which only allow one category to be selected.
Rather than spend all your time maintaining the editorial style of only putting posts into a single category, this plugin is a quick way for you to enforce that standard. It makes it impossible for a person to accidentally break your site's layout of functionality by putting a post in multiple categories. They just can't do it, whether by accident or intention.