For me, it wouldn't be a backup of backups, but more a backup of expired files off my regular backups. So I'd keep say 3 months of files in my primary backup,

For me, it wouldn't be a backup of backups, but more a backup of expired files off my regular backups. So I'd keep say 3 months of files in my primary backup, anything older (up to say 3 years) would then go to Glacier. That's the best use case of it.