Adding option for disabling and enabling USE flags like --use-write 'app-portage/hello -foo' or 'app-portage/hello foo' It is enhance usability of portage for newbies also.
You know that you can do this from the shell with a command like this: echo 'app-portage/hello foo' >> /etc/portage/package.use
And this would have the same problem as --autounmask-write, that it doesn't know in which file to write (if package.use is a directory).