While taking a backup home
with you does indeed protect
from a fire
at the office, I think it doesn't go quite far enough.
What if whatever destroys the site also destroys your house? An earthquake of sufficient magnitude could level an entire city, including your home. A tornado could cut a swath of destruction for miles, demolishing both your house and the office. Let's not even think about weapons of mass destruction...
To be absolutely safe, I'd insist on keeping a backup in a secure facility no less than 300 miles away. Preferably in a fireproof safe.