Fair Use

It seems pretty obvious that Fair Use is a critical tool for documentaries, as it protects the creator of the documentary from copyright when the source material is used for (just to list from the Code of Best Practices video) :
Commenting/critique
Illustration/example
Incidental use
Cultural rescue
Launching discussion
Mashups

Which is pretty much, like, everything that a documentary might attempt to do. (It almost seems like the only thing that fair use doesn't protect is completely ripping off an entire source, like a whole movie, and then putting your name on it and trying to sell it like that.)

I think some of the most important items on that list are commenting/critique and launching discussion. If I were to make a film (let's say a neo-noir movie that tells its own story while exploring the genre and its limitations/cliches/etc), I might at some point directly rip the scene from "Double Indemnity" (1944) when Fred MacMurray gets shot by Barbara Stanwyck (becuz he wanted to kill her, becuz they are gonna get busted by the cops, becuz they are in a scandal together, becuz she wanted to kill her husband, becuz she wanted the insurance money, becuz it's film noir). Except maybe I would extend the scene so it looks like they are shooting back and forth at each other for like ten minutes, and maybe it would also keep getting darker and darker in the frame until the end of the scene is just pitch black with the sounds of gun shots.

See there I would be making some kind of comment on the ridiculous nature of violence and how it is portrayed in this genre, and also critiquing one stylistic aspect of noir:
where it's often so dark it's difficult to tell WHATS GOIN ON.

Anyway that's just one example of what I might do, but I think Bill Maher does some pretty good stuff in his "documentary" Religulous (Which is really more interesting for his techniques at crafting an argument through film/editing techniques...moreso than for his diligent interviewing/researching/fact-ing and junk.

Maher has some pretty fascinating video clips he uses to make his points (or mostly make fun of his subjects), and I think he could definitely make the argument that his use of these clips is fair use because they comprise a critique while also encouraging discussion. And this kind of (highly entertaining) technique is only possible because of things like Fair Use that protect the documentarian's (I don't know if that's a word) use of copyrighted material.
If documentary creators weren't allowed to use any source video/audio, documentaries would be mostly blank screen and some guy talkin about what he thinks for like 45 minutes, and that sounds dumb. (It would be like writing an essay about a novel, but not being able to quote any lines from the novel, so you would just have to summarize a bunch of stuff and you would sound like an idiot).

I like this mashup because it is funny. Maybe it's making some kind of commentary on youth/aging and the ways in which our perception of the world changes as our awareness of social issues grows. Or maybe it's just kind of funny.
It's this Finding Nemo vs. Crash (that Sandy B. movie).