I have long felt that, while categories play a useful role in organizing information on blogs, tags are just useless noise. I am pleased to see that Matt Cutts, who blogs and works for Google, agrees.
From the words in your actual post, Google and other search engines can tell what it is about. Categories, on the other hand, provide a useful way for someone interested in a particular subject you write about to learn more about it. Someone visiting my blog might only be interested in security, or economics, or photography. Categories let them filter through to just that easily.