Yes, documentary filmmakers have a duty to be professional, like anyone else who is paid to perform a job. It is difficult in this particular case because of the nature of the monologue, by the cameraman should have tried harder to hold his emotions back while he was on the job site.
Yes, documentary fillmakers have a duty to be professional. They also have a duty to present real work and be human. All filmmakers are human, and if Matthew Libatique let the camera drift to cry as a result of Ellen Byrsten's monologue, that's a testmanent to her acting abilities, not his professionalism.
Documentary filmmakers have a duty to be professional like anyone else, but letting the camera drift in this case wasn't unprofessional. One of the beauties of documentaries is that they show the unique perspective of the filmmakers, who are in some cases filming deep, sensitive subjects and people. Things like the camera drift can make them even better.
There is nothing wrong with a filmmaker getting emotional, especially during the filming of a documentary. This type of incident adds to the character of the piece, showing that even those behind the camera, those that we rarely ever see or hear from, also are human. We often take for granted that there are more people involved in films than the actors/interviewees.