Another no for single categories, as it doesn't save me any work. It is not enough to know that a post is in one category, but you still need to check that it is in the right one, therefor you still need to proof read your post as it is going to be displayed and if at that stage you can not discover that you have two categories instead of one then your problems are much bigger then the number of categories.

As you described it perfectly the problem that needs to be solved is much more localized, the unintended use of default category.