I disagree with you guys. In game that may be true but in real life it just doesn't work that way. If one is performing some task (physical or mental) and something upset him and makes him angry he will lose focus and things will only get worse (I am sure everyone can confirm that from experience). It is true that at that point you can squeeze out a bit more of physical strength but usually it is worthless since anger diminishes your thinking and coordination performance. That is simply because muscles can produce more power to do some random uncontrolled motion compared to doing motion that is precisely directed by our neural system.
Fear helps push you to do what needs to be done, if you fear dying or losing then you will push yourself to stay alive and win. Not to mention been afraid of something keeps you on your feet where as been fearless makes you lazy.
Well fear is a mechanism of self preservation. It is true that it keeps one constantly alert but on the other hand it greatly limits anyone who is controlled by it. You don't do or you hesitate to do something because of fear. Many dictatorships induced fear into their population to completely diminish people's thoughts of doing anything against the establishment and keep them in line. If you fear your teacher you do poorly at the oral exam. If you are afraid of new technology you will never learn how to use computer and cell phones (older people for example). And so on.