Q: Is there a security reason why this should remain private?
I will not say that this is because of the 100% security reason. But there are other factors as well. As this Publisher ID which you have obtained from your developer account at the Store will help you create a certificate with a Publisher ID of your choice. You can also create a password.
Publisher ID helps you in signing an app package
If you either create a package for the Windows Store or associate your app with it, the Publisher attribute is set to the Publisher ID that’s obtained from your developer account at the Store when you sign in during package creation.
So it is your publisher ID which is actually identifying you. So its better to keep it secure.(It is very similar to take it like you will not like to give your bank account details to anyone except the bank ;-))
So if your publisher ID is not secure or is public then you may loose that choice.
Check this To create or use an alternate certificate