Hmm that's a tough one. I'd certainly use the argument of health and risk factors. There is a much higher risk of developing cancer as ramanth said. That is documented pretty thoroughly by veterinarians.

To the argument that you had an unaltered dog who lived so long, yes of course there's no guarantee that an unaltered dog will develop cancer, just like there is no guarantee that a smoker will develop lung cancer. BUT why let there be that kind of increased risk when you can easily eliminate it? Neutering does not change the dog's personality or make the dog lazy, as some people fear.

As doggy parents, we get our dogs immunizations to protect them from potential disease and health problems. Neutering is the same situation.

Here is a nice link about the benefits and effects of neutering/spaying, and the risks of NOT having it done. It's fairly short, easy to print out and very informative of why it is such a good preventative measure to have performed.

http://www.sonic.net/~cdlcruz/GPCC/library/neuter.htm