That's not been my experience. Optimists also tend to assume the best motivations behind the actions of others, and that will nearly always bite you in the ass in any sizeable organization.
I've been the ultra-cynic before, and agree that doesn't work either. People don't like working with you, and don't trust you.
I think we need to be realistic on order to be successful, and neither ultra-cynicism nor optimism fits the bill.
I would suggest that a healthy, reasonable amount of cynicism is a part of being realistic about how the world works.