Interesting that this topic has come up again, as it should, periodically. (Just like the oil debate?? Hehehehe)
Anyway, was talking to a pest control guy the other day while we were both in a customer's back yard. I mentioned this thread, and he made what I thought to be a thoughtful observation:
Once you've trapped a rodent in a spring trap, and you reuse the trap, after several visitors, the rest of the clan can "smell death" and will avoid the trap.
I don't know how accurate this is, but he's been in the industry 25 years, so there's probably truth backed up by experience in his comment.
FWIW & HTH. . .