Encryption raises the barrier for both third-parties and family. In case something happens to me, I want the technical barrier to be low enough for my family to discover the backup. Another reason is that in my experience encrypted data is more sensitive to bit rot and bugs than unencrypted data. I'm backing up important work stuff with Arq Backup for example and I've had my archive corrupted once. Not sure if it was the software's fault or the storage.
My rule of thumb is ... if the data should be discovered by my family in case I'm not around, then I won't encrypt it. Photos are not worth encrypting anyway, since a lot of them end up being shared on Facebook, Flickr, Instagram, etc, as photos are meant to be shared, at least with your family.
That said I still expect Backblaze or Dropbox to keep my data private. Not secret, but private and there is a difference.