The problems I see with writing my own policy are (assuming my SELinux understanding is correct):
* They couldn't automatically be used by anyone. Distros might pick them up for their own packages and admins might manually copy the file somewhere to use it (maybe replace the distro's own), but there is no good way to start using the policy automatically with just "make install".
* People seem to disable SELinux often, because it breaks some software. Having a policy isn't useful if the whole SELinux is disabled. It would be nice if SELinux had also a new mode: Globally disabled, but enabled for apps that explicitly enable it for themselves.
* The policies can't be dynamic. I might want slightly different policies depending on what my config file contains.