The decision on whether to release your work under a free license depends a lot on the purpose you write it for. If you're writing, say, a blog to express your thoughts or as a self-promotion tool (e.g. to make your name recognizable to people in your field, like Joel Spolsky, David Heinemeier Hansson, and Paul Graham), then you loose nothing by allowing your work to be disseminated as widely as possible. Maybe you stand to make a little bit of money from a particularly popular piece, but for someone like Spolsky, the income from a contract resulting from his blog's popularity is likely much more than he could expect to make from publishing a few of his blog entries.

If you intend to be a writer, you might think about things a little differently. As a given, you'll want to get paid for what you write, probably by being commissioned by a magazine for a piece or by working as a staff writer. In this case, you probably feel squeamish about giving your work away for free since you're sometimes able to make money off it.

If you're already willing to publish a piece for free on your blog, it seems unlikely that you would later be able to sell it to a magazine. Magazines seem to like original content, and (with the exception of Hacker Monthly) wouldn't want to republish something already out there.

A few people have experimented with republishing their blogs as books. The two that come to mind are PG's Hackers and Painters (his essays weren't exactly a blog, but they're pretty similar in that they're regularly updated freely accessible online content) and Scott Adams' Stick to Drawing Comics, Monkey Brain. From what I've gathered, both of these had limited success, making this strategy of generating income from blog content seem rather unfeasible.

The only remaining strategy for making money off your blog is ads. This has certainly been proven to work, although only for blogs in the top one percent or so in terms of readership. Boing Boing, the most read blog on the web, licenses it's content under a Creative Commons license, and it doesn't seem to have hurt it much. XKCD also releases all its content under a CC license. (They both use the Attribution-Non Commercial license, which does raise some questions about what would happen if they removed the non commercial clause.)

So, if you're not in the top one percent of blogs, it seems like you don't have much to loose by giving your content away, and having it available in as many places as possible. You can turn the notoriety you gain from people listening to your thoughts and ideas into something profitable if you like, such as promotion for contract work, publicity for a book you write, or even starting a startup incubator. Don't get caught up in the details of the value of a few blog posts, think big picture.