The ones that truly can see the future or at least get a glimpse are more than up front to tell you that what they see is only one interpretation. The future is fickle. A change in a single decision can create far reaching changes.
When a person tells you about an event that hasn't happened yet, then you have created a change in the time stream and reality. Some events are so large and far reaching that knowing something about may not be able to change it.
Having a vision of World War III doesn't mans there is anything you can do to stop it. Something like has so many thing pulling reality towards it that the knowledge of a single person may not be enough to stop it. Especially since the knowledge people glean of the future is often confusing by itself. It's just a few fleeting glimpses and then nothing.
Knowing the future creates an interesting paradox, does knowing the future automatically change it. I mean, if someone told you that you were going to be hit by a car at 1:45 p.m. on Tuesday, then you would make a point to stay in.
Also, does that mean you have beaten fate or only postponed the inevitable.