Again... how does this model fatigue? Fatigue usually has an impact on one's ability to perform tasks... In 3.5 "fatigued" is a condition that causes a character to take a -2 to Dex and Str...which has nothing to do with hit points, it affects a characters performance and requires 8 hours of rest to remove the condition... and if the character does anything else that would cause them to be fatigued they become exhausted instead.
Loosing a surge has no effect whatsoever on your immediate performance (you are just as good at doing everything as you were before loosing the healing surge) thus I am asking what part of this models being fatigued in any way?
You are correct, most representation of fatigue in games impose a negative penalty to doing future tasks - thereby leading to a higher chance of mistakes and fumbling tasks (in many games the penalties are cumulative thereby leading to the death spiral mentioned above). While this is one of those things that intuitively makes sense, it's not necessarily accurate. Google up some studies on death spiral effects and you'll notice they mostly show a fight goes like this: fine, fine, fine, down/dead.
Healing surges model this form the point that eventually you run out of reserves, your body can no longer support the healing necessary to keep you going - rest or fall down/die. The character is not impared from a bonus/penalty perspective but is certainly impared from an effectiveness perspective and the loss of effectiveness can be attributed to fatigue as easily as anything else. It forces the player to make hard choices on continuing in a diminished capacity without punishing the player further with the imposition of a death spiral (I dislike death spirals in adition to not being that accurate they are no fun at all).
What exactly is the downside to being fatigued in loosing a healing surge?
In losing healing surges you are incrementaly less effective, even if your HPs are the same with 9/10 Healing surges you are pretty close to normal, 5/10 your ok but have to consider resting in the future, 0/10 you may be functional but you are nearly walking dead, sure you don't have minuses but effectively every action you take has greater consequences because your reserves are so low and not resting risks serious injury or death.
You are still able to perform all actions as if in tip top shape.
Not really, you may not suffer minuses but since your actions have more drastic consequences (HP loss without likely healing for example) you are effectively impared.