Without a doubt, writing a spec article and having it accepted is more difficult than writing an assigned piece to a magazine's specifications, with the angle designated by an editor already committed to the purchase. However, a talented and persistent writer can do it. He may even prefer to do it. As mentioned, it builds a resume. I view spec writing as paying my dues, a necessary stepping stone in my career.
Spec writers can pick and choose topics, selecting subjects which fulfill personal needs.
Learning to do it, then writing an article about the process and pitfalls, can serve both you and your writing career. Use the material of your life for a how-to on hiring an attorney. A part-time freelancer with limited expendable hours will find this dual use of time efficient and rewarding. You write on your own time, at your own pace. Assigned articles inevitably mean deadlines.