Agreed, local storage is good, but a cloud back-up will prevent loss if say, your house burns down, floods or if somebody breaks in and steals all your stuff. That happened to me one time. Had a break-in and they took all my electronics. Fortunately, it was a new computer that I didn't store photos on.
I am guessing if you have 2 external drives, then your are reducing the chance of loss due to failure.
I have been using Crashplan, but they have changed the rules a couple times. Had to upgrade to business, then it wanted me to update an old computer to continue keeping that back-up on that one. Considering looking at another cloud back-up. Considering how California seems to be burning down around me, I really need a reliable cloud service.