Backup policy on a standby database

I'm just reviewing the backup policy on some of our databases with associated standby databases.

All the primary databases are set up with nightly RMAN hot backups including backing up the archived redo logs.

Some of our standbys are set up to backup the archivelogs only, and some don't have this.

If we back up the archive logs on the primary, why would we need to backup the archivelogs from the standby, so I was just wondering what the Oracle best practices are in this area - should we back up archivelogs on standby databases or is this overkill?

thanks