>> What I don't fully understand, however, is why prohibit state-changing
>> operations in Dired buffers. Is it just because there's no VC-Dir buffer
>> easily at hand, to update the visible file statuses?
>
> Exactly the same question I ask myself every time I forget about this limitation
> and type 'C-x v v' in Dired.

I see now why it's difficult to support state changing VC operations in dired.

The difficultly comes from the need to get a VC state from every marked file
and to confirm that all marked files are in the same state.

So I implemented this in vc-dired-deduce-fileset whose basic
algorithm was taken from vc-dir-deduce-fileset.

Now with this patch state changing VC operations are supported on marked
_files_ in dired.

However, the need for the OBSERVER arg still remains because this patch
doesn't support state changing VC operations on marked _directories_ in dired.
It raises the error in this case:

  State changing VC operations on directories not supported in `dired-mode'

State changing VC operations could be supported on directories as well,
but there are several possibilities to choose from, and I can't decide:

1. On a marked directory get a state from all its files
   (probably this variant makes no sense)

2. On a marked directory get only files with a "modified" state
   (i.e. edited/added/removed)

But the latter is not easy to implement because the needed functionality
is missing in VC, because the only VC method that is suitable for this is
'dir-status-files' to get VC files from a directory but it's too tightly
integrated with vc-dir and can't be easily separated, i.e. there is
no easy way to do something like:

  (with-temp-buffer
    (vc-call-backend
     (vc-responsible-backend default-directory) 'dir-status-files default-directory nil
     (lambda (entries &optional more-to-come)
       (message "! %S" entries))))

Once we decide how to support marked directories in dired, the arg OBSERVER
can be deprecated.