If a pet is stolen it's irrelevant if it's chipped or not. We've been searching for our missing cat for almost 9 weeks now, he's microchipped but unless someone takes him to a vet/rescue and says they've FOUND him, he won't automatically be scanned. As far as I'm aware most vets only scan found or RTA animals, not new pets. That's where the real work needs to be done, ensuring that all newly registered pets are scanned on their first appointment with the vet. That way they would find out immediately that the pet was missing (assuming the owner had done as we have and reported him missing to the pet database).
But I agree with the OP, all pets should be chipped (I even suggested a colleague get her rabbits chipped today!)
Gem.