This is actually a very good point, and can be applied to the Force. Anger and Fear, while leading to power - also lead to losing control, and losing efficiency. Unless a Sith can learn to control his fear and his anger, and hone its raw power he/she would lash out in a blind rage. In such instances a Jedi, or other opponent, can exploit their weaknesses in their sloppy and uncoordinated movements.
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.
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.
This is a purpose of Dun Moch, by enraging your opponent you can more easily exploit their weaknesses and anger as fear take over skill and discipline. However it can lead to the 'boomerang effect' when they use those emotions to overpower their enemy.
But yes, IMO anger and fear are as much weaknesses as strengths. If you cannot learn to control them they can easily become a double-edged sword.