I've struggled with this, too, both in my web work and work for clients. What I've come to is, yes, you can use both, but only if the categories are tightly con

I've struggled with this, too, both in my web work and work for clients. What I've come to is, yes, you can use both, but only if the categories are tightly controlled. For example, a site could have main categories that describe the type of content (essay, photograph, video) and then use tags to describe what the content is about. That way, the content is sortable by type (ie category) and content (ie tag).