On Friday our local animal control brought a severely injured dog to the veterinary clinic I work in, he had a puncture wound that nearly hit his spinal cord along with a huge laceration along his back and neck. The dog was not wearing a collar and a microchip was not found when he was scanned. The question on everyones mind was "how aggressive should we treat"? When we do work for city animal control it generally means we give the minimum care (make the animal comfortable for the required three days). The problem is we see these injured animals and want to do whatever it takes to make them better but there are limited funds when it comes to treating pets without owners.
This poor dog got the best we could give him but with no identification we could not get in touch with an owner (assuming he had one) and the dog had to go back to the shelter to wait out the required three days. The veterinarian has the right to recommend euthanasia if he/she feels the animal is suffering and can not be made comfortable with medication. It is a very difficult decision to make especially knowing there might be an owner out there.
My point is, ID your pets. If they get loose and are injured animal control is not able to fund all necessary treatments, therefore your pet may not get all the care it needs to make a full recovery. If that thought is not scary enough, if your pet does not wear proper identification and were to go missing you may never be able to find them.