I don't think that's how the world works, you don't go from "I blogged at the Huffington Post" to President, but maybe it gets you a paid journalism gig, or a book deal, or it just fractionally helps the pagerank of your own site. Maybe it just looks slightly more interesting on a résumé than your wordpress blog. What do you expect to get out of writing about your interests in your free time?

Anecdotal example: I'm essentially a nobody but was approached by a literary agent after a HP blog post.

> I don't think anyone sees "HuffPo Contributor" as a byline and considers the author to somehow be a legitimate writer.

Well you'd be wrong; writing on there regularly lands people on political talk shows and helps them gain visibility as a writer.

HuffPo should not be a writer's full-time job. It should be used as a platform to drive traffic/attention towards something else that you're working on.