My vet will check most dogs automatically until he knows the human/dog relationship is "true". He simply states it's to make sure the chip is "still working"
One of the very first things to do if you lose your chipped dog is to call the company that has the chip records and TELL them that the dog is missing/stolen or whatever. They will tag the record. If someone tries to change it or questions the chip, they'll let you know. It would be *perfect* if they tried to change it ... then you'd know exactly who has the pup to get him back.
ALL of my animals are chipped, and have been since chipping became available. It's a rather inexpensive layer of security that you hope you never need.
I think it was on here that I read the story of a woman who had her pup stolen at a PetSmart or what-have-you. Right from the basket and out the door - movin' fast, and no one could stop them. Several weeks later, the lady just happened to be back at the store and saw someone with her pup. The dog, of course, was overjoyed. The woman holding him said he was just friendly. The first lady demanded that the police be called and the second woman and dog were detained until their arrival. The first lady said "he's my dog and his chip number is *****". And it was. So guess who went home with the pup???
All I can tell you is that it's a layer of protection you have that you won't have if you DON'T chip them.