Conversely suppose that you had a dynamic group and you wanted to exclude a few people was a one-off report. Obviously I remove those excluded people and run the report. What I then want to be able to do is put everything back to the default dynamic group. The last thing I want to do is have to include back all those I have excluded.
Another solution would be to be able to create a static group from a snapshot of a dynamic group. So in the above scenario I would copy the group, exclude the various people, run the report and delete the static group. Leaving the dynamic group untouched in any change to its definitions.