Many times a first time speaker begins with something like...
"well, uuuhm, you know, the first time I saw Bob, uhm, you know, I was, well, surprised."
Fixed in post, this quickly becomes "The first time I saw Bob, I was surprised..." Cutaways hide these edits. 9 times out of ten, this is why you catch it as a split edit. Also, a viewer will always rather see Bob here than some dude babblin' about Bob. It's a natural fit to cover with Bob.
There are no rules here. That's what is great about documentaries. You get to make em up on the fly. In the end, making the best viewable product is the art. That quite often means making a nervous dude comfy in post.
