- If it's not off-site, it's not backed up. I was once burglarized. They took my computer. Yay, backups! Wait, no, they took my external hard drives, too.
- If it's not automated (and by automated, I mean ZERO human interaction, not, "my backup is automated but I need to move my disks once a week"), it's not reliable. It is inevitable that the day you get a failure is the day you're at the tail end of your manual rotation cycle and you just happened to lose some photo that you really liked.
- If the off-site backup medium isn't online (that is, attached to a live computer where failures will be detected), you need to have two copies. Also inevitable, that you will need your backup, and you will plug the drive in, and you will hear click...click...click.
If you're an average user, a copy on your computer and a copy with a verified backup service like Crashplan/Backblaze is enough.
If you have important data, a copy on your computer, a copy on a second disk/external drive, and a verified backup service is enough. This is what I do. I pretty much assume that the chances of two drives failing simultaneously AND Crashplan's datacenter going up in smoke at the same time is slim.
For the paranoid, you can scale further out - one on your computer, a copy on a second disk, an offline/offsite backup, even two cloud providers.
Pay the $5/month for Crashplan's unlimited service. It's easy, it's cheap, it's automatic, it's got great file versioning capabilities, and your data will go into a real datacenter, not your buddy's laptop that he regularly perches his beers on. If security is a concern, you can protect your data with an encryption key, but protect that key with your life because your data will be irretrievable if you lose it.