You may be in trouble if the plumber you thought you just called is really a handyman in disguise. Sometimes a handyman can do a great job with your plumbing needs, and sometimes he can't. Either way, you're going to be stuck with the bill.
A licensed plumber will be acquainted with local building codes and regulations. In many states, he'll also have completed a certain amount of hours on the job, so you know you're getting some experience. Finally, a licensed plumber is fully insured in case something goes wrong or somebody is injured on the job.