They can be, in moderation. By moderation I mean, adding your blog or Hubpage manually and progressively to directories over time, not by using mass submission with software etc. Google will penalize you for that, because it's unnatural linking. Adding your site to "relevant" blog directories is a good way to gain awareness for your site, from the search engines point of view, as far as traffic goes, you probably won't get much.
To find relevant directories to add too, go to Google and type in your 'keyword', ( the niche your blog or page is related to) then type directory. Like this, "golf directory" substitute 'golf' for your own keyword, it should bring up thousands of directories for you, free and paid. Pick the best ones to use, though some will ask for a reciprocal link back to them, you can skip those and find others that suit.