There is a high possibility that Google's server is 10 times more secure than any average Joe's mail server. Even more secure than a security expert's server as well: http://www.darknet.org.uk/2009/08/dan-kaminsky-kevin-mitnick...
Do you have time to manage and keep your server secure? Can you patch a new 0day in an hour, possibly not.
On the other hand when men-in-black asks to Google yes they might give it away, which is another story.