I've been freelancing for more than 10 years, proofreading and copy editing in advertising, medical advertising, publishing and legal. Being a freelancer has its pros and cons, however. By the way, I've been at the same place now for going on four years so it does not always mean jumping around.
Pros: flexibility to pursue outside interests, exploring the workload and culture of a company before signing on, getting an edge on being hired in a place you like because you are a known quantity already (inside instead of outside), more security when companies are downsizing (freelance is less expensive than staff - no health care, no vacation pay, no sick pay).
Cons: NO Health Care, no guarantee of hours, variable cash flow (7 hours up to 60 hours a week), work at many companies or one, instability and uncertainty, less security unless it's a permatemp position.
So why do I do it? I enjoy meeting new people and engaging in different work projects. I also have many interests outside of work (tutoring, writing, real estate, internet businesses, meditation, travel, etc.) that freelancing's flexibility allows me to pursue. I like the agility of keeping a pulse on the workplace and market, learning new skills constantly to make myself more marketable. When there is little work, the downsides are formidable but I wouldn't trade it for anything because I yearn to make a difference, change the world. For that, I need the free time that freelancing gives me.