Of course not. You don't even have to have an agent to be a professional writer, or a successful professional writer. Writers have successfully represented themselves before, and if you're a lawyer with previous experience in the publishing field, you are probably safe representing yourself. The old saw about the doctor who treats himself having a fool for a patient, however, is not without its applications here. I would be much worse off professionally without my agent.
Good agents do much more than find homes for manuscripts.
If he did nothing more for you than remove bad clauses from contracts, he'd be worth his ten or fifteen percent. You know this because you are going to take my advice and find both a reputable agent and a reputable publisher, and because real agents and real publishers are in the business of getting good books into print. They wouldn't fool around with plagiarism any more than they would voluntarily contract bubonic plague. Also, you might want to click here to read about the other reason that professionals don't steal ideas. Russell Galen of Scovil, Chichak, Galen.