I believe all cats should be spayed or neutered unless they are owned by a licensed breeder. This is because in many countries there is an overpopulation of feral cats. In just seven years, one cat and her offspring can product about 420,000 kittens.
There are 11 million cats and dogs euthanized every year. It breaks my heart just thinking about an animal being killed just because it doesn't have a home. Some people believe that when you fix your animal, it will become overweight but this is a false statement. It actually cost less to spay/neuter your pet than it does to take care of a letter of kittens or puppies.
Altering your pet can prevent testicular cancer, pyometra and breast cancer. Spaying/neutering your pet prevents females from going into heat and males from roaming away in search of a mate.
By allowing your cat to breed, you've increased the number of cats that will have to be euthanized because no one wants them. I believe there should be some sort of law about spaying and neutering your cat and dog so this overpopulation does not occur.