The article brings up good points for and against encrypting backups, and I agree it warrants some careful thought.
One (minor) additional point in favor of encrypting backups: when a HDD becomes unreliable or fails, you don't have to worry about people getting data from it, even if you're unable to do a complete secure erase or destruction of the HDD before disposal.
If you have trouble getting the drive to work, a secure erase may fail or only partially work. But I think it's plausible that someone else with more sophisticated software and hardware tools might well be able to copy data from it. Presumably in that case one would disassemble or destroy the HDD in some manner. But how many people will really do that?
That said, I'm more concerned about losing access to my data (mainly photos and home video files) than I am about someone else getting it. Although I can't quite come up with a scenario where a backup's encryption would prevent me from restoring my data, I have a nagging worry that it's possible in some way I haven't thought of. So for myself, I lean toward having at least one unencrypted backup.
Edited to add --- All the data that I consider could be seriously abused (e.g., personal financial records) I already store in an encrypted disk image. So that material IS encrypted even on my unencrypted backup.
