I've heard from Tiffanys that they use platinum in their rings because it is stronger and will keep, especially smaller, diamonds in so there is no chance for one to fall out.
I've heard platinum is stronger too. My DH had my wedding ring custom made to be "industuctible" since he knew I am hard on rings so the jewler used platinum. I have a lot of scratches and dents in the platinum but the diamonds are still very secure.
One downside of platinum that I didn't see mentioned here is that platinum feels heavier than gold.