I just landed a sweet content-writing gig! And I never would have gotten it if I had followed my initial instincts and refused to write on spec.
Lots of writers will tell you not to work on spec, because you put all your sweat and time into an article that has no guarantee of being accepted. And in some cases, that's true. The first query letter I ever sent out came back with a request to see the completed article on spec. I spent a month outlining, researching, interviewing, and writing...only to never hear back from the editor after I sent in the completed article. That's frustrating, and I can totally understand why a lot of writers advise against it.
Sometimes, though, it's worth it. For example, the gig that I just landed is an ongoing project...2-10 articles a week, and a decent pay rate for each article. That translates into good things for my bank account. So they needed me to write a spec article to prove that I could provide quality content. So what? The article didn't take me too long to write (I researched and wrote it in a couple of hours), and the payoff is well worth it. If they'd turned me down, it would have been disappointing, but I would have had a decent content article (plus a ton of story ideas on the subject) that I could have sold elsewhere.
Like anything in this business, you have to do what works for you. As far as my business goes, I no longer write for free (unless there's another perk, like a free book or CD from review sites), and I won't respond to insulting ads, like $1 for a 600-word article. But from now on, if the potential reward is high enough, I'll absolutely write on spec...at least until my résumé is impressive enough that I don't need to anymore.
Now excuse me, I'm going to go pop a bottle of champagne and celebrate.