Are people still backing up their primary instances if they have backups of their dataguard copies?

A company has an 12c RAC production environment with a 5TB database.  The production RAC database has several dataguard copies that are constantly kept up to date.  Using RMAN, that company takes regular backups of one of the dataguard copies with the understanding that this is the equivalent of backing up the primary production environment.  One major justification for this is that there doesn't appear to be a reason to put a load on the production hardware to run this massive backup since the dataguard copy should be identical.

Is this reasoning sound?

If the production database crashed, could they recover the production instance using the backup of the dataguard copy (testing appears to indicate yes) ?