Gross inequity is undemocratic. I cite Kevin Phillips' book Wealth and Democracy http://www.amazon.com/Wealth-Democracy-Political-History-Ame... but it's been researched and detailed ad nauseum.
More equitable societies are happier.
More equitable societies have better public health.
Just in case you need cites: https://www.google.com/searchq=More+equitable+societies+are+...
Lastly, I believe, but cannot prove, that more equitable societies are safer. Both domestically and internationally.