I'm with Desperado on this - our city code is against double cylinder locks, but lo, there's one on our back door, probably installed by an earlier homeowner or pre-code. We keep the key in it 24/7/365. A door lock - any door lock - only provides $25-100 of security, or at most 30 seconds of time until someone gains entry. You are better off locking up your valuables with a much better level of security than your door locks can afford you. Personal protection - there are options there too, including alarm systems. On the other hand a double cylinder without a key in it can prevent egress of my wife or kids in the event of a fire, human loss = priceless. I know which side of the equation I come down on.