The Olympics were ostensibly formed under the guise of amateurism. But what, exactly, IS amateurism?
If an amateur is a person who engages in an activity, especially a sport, on an unpaid basis, then wouldn't by nature every participant at the Olympic Games be considered an amateur? Not a single athlete is paid by the International Olympic Committee or their respective national federations to represent his or her nation at the Olympics. So by the basest definition of the term, professionalism per se has no impact on the Olympic movement...
But this question delves deeper than mere definitional debate. Where the abstract notion of amateurism developed plays a large part in how people today perceive the Olympic movement and the advancement of professional sportsmen and women to the medal podium and the gold medal. A derivative of the English school system of the nineteenth century, codes of conduct and amateurism was developed from their natural forbears in Greek philosophies of athletics as an extension of education. Mens sana in corpore sano: a sound mind in a healthy body...
Athletics, however, proved too great to be merely restricted to those who could afford to enroll in elite schools. Not merely the best and brightest were capable of showcasing physical prowess. And, furthermore, there was much profit and enjoyment to be made from the spectating of these sports spectacles. Therein lies the connection to another bygone culture - that of the Roman empire and its history of placating the masses through bread and circuses.
Not every locale, however, grew a sports culture in the Victorian vacuum of decorum and notions of playing games to become better educated. The Olympic movement was formed to showcase the burgeoning desire for great feats of athleticism, yet even as the first Olympiad took root in Athens in 1896 a larger movement was growing in sports to commercialize the events. Cycling had captivated audiences on both sides of the Atlantic, with bicycle companies swooping in to sponsor riders and teams in droves in road and track races. Baseball grew in stature from the beginning in the United States as a professional enterprise, setting the tone for the current American sports landscape, where young athletes pay lip-service to college (or eschew school altogether if league bylaws permit it) and bolt for the professional ranks at the first chance. No longer are athletics an extension of a holistic education; now education is a necessary evil to strike it rich in the big leagues...
Even before professional athletes were accepted in full at the Olympics in 1992, there has been a long-standing tradition of athletes getting paid to train for international amateur competitions. Sports programs in the Soviet bloc were designed to preserve athlete's amateur eligibility, providing them with "jobs" or "education" that turned out to be little more than a smokescreen - the reality is that these athletes were getting paid to represent the nation in World Championships and Olympics and other intercontinental competitions. Likewise many stars were able to accept sponsorship offers from clothing and equipment companies... not getting directly paid to play the game, but rather to model the wares of a corporation. Amateurism is a blurry line at the finest of times; before the IOC stepped in to permit all the world's best to perform back in 1992, some nations which worked actively to maintain amateur eligibility for their best and brightest invariably had the advantage ever four years...
Initially, the professionals had the run of things. With the natural conditioning which comes from regular high-level competition and world-class training facilities and coaching staff and medical personnel, the professionals were able to trounce the part-timers. However, instances like the United States' "Dream Team" in basketball have become increasingly rarer as the exposure of the world to the greatest talents has inevitably bridged the talent gap as more international players are traversing the globe to find better competition. Some sports, like track and field, have encountered issues of doping as the lucrative sponsorship windfall which follows a gold-medal victory leads athletes to seek an artificial edge - but doping was pervasive long before professional athletes were welcomed into the Olympic village, and no restriction on eligibility will prevent their continued presence.
Professional athletics do not harm the Olympic movement. After all, the Olympic creed - Citius, Altius, Fortius - demands that sports strive to go swifter, higher, and stronger into the future. The preeminent athletic competition deserves the world's best athletes, and the natural movement away from archaic nineteenth-century principles of amateurism and into the twenty-first-century reality of where the best athletes compete only serves to provide those athletes every four years.