People wonder why I rotate 3 external drives on my boxes and do weekly system image backups, keeping the last 3 backups on each drive (giving myself 2 months of weekly restorable images).
It takes me maybe 1 minute a week to plug up the right drive, turn it on, fire off the backup, and then disconnect when done. That's under an hour a year; well worth my time, w/ external drives so cheap.