Both Magnus and Zoey got chipped when they got neutered/spayed. Just like what yorkietalkiejilly said... if it gives me 1% or even .05% chance of getting them back, I think both myself and pups won't mind the few seconds of pain.
In the US, dognappings have been on the rise. Bad economy = high rate of dognapping. So always keep an eye on your pup. Because the purebreds and small breeds are the more popular ones these criminals target. The scam is, dognap the pet and when flyers/posters/ads are out offering reward for the pet, the perp goes to the owner, gives back the pup and takes the reward. Giving any number of excuse like "Oh, I found him roaming around the Walmart parking lot."