This was discussed at the Scottish Parliament Cross-party group for animal welfare during a presentation by Dogs Trust who want compulsory chipping for all cats and dogs. Chips may migrate in animals with low body fat ie horses, greyhounds, some pups, and as you point out, can pose problems if they migrate to joints particularly. The DT rep claims chip migration is fairly rare, and one way to avoid this is by using intramuscular chips, which horses have, which obviously must be carried out by a vet and is a more complex procedure than standard chipping. However, compulsory chipping of racing greyhounds has been introduced in England/Wales, and it will be useful to monitor their experiences of this, whether it leads to more hounds being discarded, or indeed unscrupulous removal of chips along with tattooed ears.