I've heard a theory that it makes people feel better about their own helplessness.
That certainly sounds like an accurate theory, and I have been known to be cynical about stuff every so often, and am still trying to remove that from my personality. It may have been a defense mechanism developed to block all the helplessness, as that theory describes, but that does not justify my demeanor in those instances.
Like you, I believe having an optimistic stand in front of all the adversity, if nothing else, makes things a bit more manageable. And I agree that if more people dropped cynicism, the world would be all the better for it. Having said that, once that's embedded in a person's character, removing it is an entire re-learning process which requires one to forcibly unlearn it, because it does turn into a subconscious reaction.