Not everyone has access to decent reliable internet and git is pretty much useless if it gets interrupted. I would prefer to be able to clone the source code[1] without the documentation without having to resort to some weird git commands to get it done. Of course, having git bundles available over resumable http(s) can help a lot with that, but they are almost never provided.
Just about any use case is going to pull other repos for payloads, etc. anyway, so I don't see why having the documentation seperate would be a problem.
[1] I already have the coreboot and board status repos cloned, but that doesn't change the fact that I had issues getting it done at first. Board status was particularly bad, over 200 MB to clone, over a gig as uncompressed text. all just for the script to upload a new commit - it shouldn't need all that to do it's job.