FWIW, as a user I think this is the expected behavior. The option is named --add-missing, so if the file is not missing, I expect nothing to happen.
I more or less agree, but the original suggestion from Ben (https://debbugs.gnu.org/cgi/bugreport.cgi?bug=33573) was for --add-missing --copy to overwrite existing symlinks with files, because of the --copy. So the current behavior of silently ignoring existing entries doesn't suit everyone. But I don't think silently overwriting symlinks is right either. I can see that specifying both --add-missing and --copy leaves it rather indeterminate what should happen with existing symlinks. In practice, I doubt people would want to end up with some symlinks and some files for the "missing" list of files, which is what would generally happen now. Hence my idea of simply reporting the situation (not failing, just warning), and if they want to overwrite symlinks with files, also use --force. It occurs to me that, in the analogous situation from the other side, when --add-missing is given without --copy, one could argue that a warning should equally be given if a file (not symlink) exists. But somehow that feels like a step too far, although I can't think of a real argument against it. --best, karl.