Facing the camera, while disconcerting and rare, can be a useful tool in documentaries. Errol Morris uses it frequently to great effect. The difference in appearance creates two types of docs: looking askance (at the interviewer aside the camera) is best if the effect is to be the impression of an impartial filmmaker. It's clear the answers are the interview subject's opinion and the film is a collection of these opinions. Audiences can then make up their own mind at the end.
Looking directly at the camera, as if the audience were asking the questions, gives the impression of absolute truth (whether or not this is the case). The subjects are all-knowing and there is no matter of opinion. These collected interviews aren't a collection of opinions; they are absolute truth. You don't often see, it's seldom used wisely, it and it should be used sparingly, but the effect is huge.