What if your Ghost backup fails?  Of course the external drive could fail mechanically.  But another possible weakness in a nightly snapshot of your system is that it doesn’t protect against a gradual corruption of your system by say adware or spyware. If you find your system is messed up and you want to restore it to a clean state you would not want to restore yesterday’s snapshot (of this corrupted configuration).

If you were careful enough to create and save a snapshot of your system that you know is clean (from say 6 weeks ago), you could quickly restore the clean configuration of 6 weeks ago and then just recover from your nightly backup any data that has recently changed.

Another option is to buy two external drives as your backup devices. You can take a snapshot of your clean configuration onto drive “A” and stick it on the shelf (or in a safe deposit box), and then you use a second drive “B” for your nightly backup.  But it is also more pricey since you have to buy double the hardware.