Neutering in rats slightly increases lifespan of the animal. If this could be proven in dogs/cats, etc. as a more general biological phenomenon I think it'd be a great selling point for the procedure to new pet owners.
"New research on the biology of aging in dogs suggests a link between shortened life expectancy and ovary removal."
"The study, published in the December 2009 issue of the journal Aging Cell, found that Rottweilers that were spayed after they were 6 years old were 4.6 times as likely to reach 13 years of age as were Rottweilers that were spayed at a younger age."
Who doesn't want their new pet to live longer?
http://www.ratbehavior.org/Neutering.htm