Idealism is necessary for someone to behave with honor. I am wary of people who are very up front about their own cynicism – I think it’s expectation-setting behavior. It may be presented as a form of humility, as in “I’m very self-aware about the limitations of my own understanding”, but what I’ve seen it come out to mean is “Don’t expect me to hold to any of my commitments, as I will plead ignorance for any acts of self-interested malice I willingly commit.”
Necessary doesn’t mean sufficient, of course. There are plenty of idealistic assholes out there – it’s just that the self-labeling cynic is declaring their intent to be an asshole upfront, and one does well to take the free warnings as they come.
Inspired by the bit about expectations, employment and marriage in the most recent EconTalk: http://www.econtalk.org/archives/2014/08/reid_hoffman_an.html