I guess I just don't understand how it is properly used. That would mean
it needs better documentation or maybe a change of name.

1. Summarise the issue (what were doing, what went wrong?)

What is the use case for saying

  darcs log README --no-only-to-files

and not just plain

  darcs log

? I mean, I can just give it no filenames when I want all the patches,
right? And if I give it filenames, well I don't need to say that
explicitly by giving the --only-to-files option.

And it doesn't even work: darcs log shows only the patches related to
the given file(s) even with --no-only-to-files.

2. What behaviour were you expecting instead?

Well, darcs behaves exactly as I would expect, only it has this
apparently useless option pair that doesn't do anything.

Or rather, I guess it does something (the code looks as if it does), but
from the outside I can't figure out what that might be.

3. What darcs version are you using? (Try: darcs --exact-version)

2.8.4 or later

Ok, I finally found out what this option is good for. It is for darcs
changes, and it means that darcs should show only what has changed in
the listed files. W/o --only-to-files, only the patches that touch the
specified files are listed, but the diff is shown in full i.e. for all
files (with -v or when selecting v interactively).

In fact a very useful feature that deserves better documentation.