Counterpoint, I have on occasion found category pages on wikipedia to be enormously useful. I'm not aware of any other way to see in one stroke a huge chunk of specialist vocabulary associated with a single topic. Real-world use case, I was looking for a good name for a project that focused on security. http://en.wikipedia.org/wiki/Category:Medieval_defences is a treasure trove of rather more obscure starting points in a way that http://en.wikipedia.org/wiki/Castle is absolutely not. An alternative use case would be a choice of Java or .NET as a platform, a list of available languages is absolutely helpful in that case.

Category pages are useful as a supplement to good (not-deleted) primary pages.