Over time, data storage decay. As we can’t assume it will always be stored in a self healing manner, I would put such a self healing mechanism in the backup itself. That being said, it might be overkill in the sense that most data storage media already implement an error correction mechanism (SD, HDD, SSD, optical disks). Still, each mechanism has his own tolerance for failure, and sometimes the error correction assumes regular use. To answer your questions: backups on an offline Flash, Magnetic or Optical storage, let to data rot for 10 years. Sure, extra ECC won’t prevent failure ultimately, but could prolong the viability of the data long for a negligible cost.
While having more backups is the better option, it’s still not certain the user will easily be able to store and recover data in a large number of places. Another point would be that a user might not want to leave copies everywhere (or in public, even if encrypted).
To conclude, I’d say that this scheme should still “strongly recommend” having multiple backups, in multiple places. But any one copy should also be pretty resilient against decay, more than your average data.