I totally agree with you, but then it would be better if category XXX is always last to show, so the most interesting categories are always on top of the search.
Our use-case:
Every time our web-application creates a project a post is created announcing the creation, this allow us to have comments in that project inside our website. Then we have a real forum with way more interesting categories where people are usually active in topics not always related to the projects. and then when you have 1000s of projects then you have a lots of posts that “real forum users” are not interested (95% of time).