thekestrel,
My mother always tells me that locks are for honest people. By that she means a determined criminal won't be discouraged by a little security. With that said, there are very good reasons to not leave your doors unlocked. While there are ways to minimize threats, some risk must be assumed.

When I said I didn't have a "good" answer I was indicating that I wasn't aware of a generic one-size-fits-all solution. Given a specific set of conditions certain choices make more sense then others as indicated by other responses in this thread and elsewhere.