Snapshot controlfile

My understanding is that a snapshot controlfile is created by RMAN so it can take a backup of a consistent copy of the controlfile without acquiring locks on the controlfile. Since controlfile is always being written to, RMAN creates a snapshot controlfile. Once the snapshot controlfile is created, no Oracle process should be writing to the snapshot controlfile while RMAN backs it up. Is this correct?

In 11gr2 RAC, Oracle mandates that the snapshot controlfile must reside on a shared disk so all instances have access to it. If this not done, ORA-00245 error is raised during backup of controlfile. In my view, having a snapshot controlfile on a shared disk is contrary to the purpose of the snapshot controlfile.