The question you need to ask yourself, twitchboy, is what kind of game do you feel you are playing? Are you playing a boardgame, or are you playing a roleplaying game?
If you treat D&D combat like a boardgame (which many people do, and there's nothing wrong with that)... then sure, I can certainly understand why the fudging of rolls would be bothersome. In boardgames, the players are trying to 'win' the game through legitimate means. You wouldn't want dice fudging in Monopoly or Risk, so why would you want it here?
However, if you treat D&D combat as part of a roleplaying game... the central aspect of an RPG is 'story'. Every combat tells its own part of a larger story, and the hope is that every part of the story is interesting, compelling, and exciting to witness and participate in. However, the DM knows quite a bit more than the PCs do about the story that a particular combat is a part of... and if fudging a roll here and there helps maintain or even raise the excitement of the story, then in the long run it usually makes for a better game.
So for instance:
Your Ranger just Criticalled on the big monster and did some ungodly amount of damage that brought him down from not even bloodied to 2 hit points. The DM decides to wipe off those last 2 HP as well in order to make this huge attack be a massive killing blow... rather than have the monster take the shot but then wimper on the ground for a bit with 2 HP just to wait for another PC to walk over and finally clonk him over the head. Bit anticlimactic.
Or:
The party is climbing a mountain face to get to the dragon's roost and through a series of really bad rolls during the skill challenge, one of the PCs fall from the climb. And just through extreme sheer bad luck, the damage roll for Falling Damage is just so high (much higher than what the DM really expected to have occur for this encounter) that the PC goes directly to negative bloodied and is instantly killed. Now the DM planned for the climb to be treacherous and for the PCs to need to expend resources on their way to the dragon's roost, but not to completely eliminate one of the players before even getting there (since rather than continue onto the roost, the party would probably turn around and go home in hopes of getting a Raise Dead). So he fudges the damage roll so that the PC does not actually get insta-killed, just horribly maimed and lying on an outcropping halfway down the mountain. The PCs now have several compelling decisions to make rather than what would have been probably the single one of "go home".
Whether either of these decision points are worth 'fudging' or not are open to individual DMs and what they think would make for good drama (like I'm sure there are some DMs who would consider the death of a PC from a fall more interesting dramatically than having him maimed and unconscious on a ledge halfway down). But they do illustrate points where fudging rolls due to really unexpected results can help provide possibly better story or 'moments'. And at least in my opinion... when a PC gets to experience a 'big moment', or have to make a hard decision... that's when compelling and interesting drama or joy occurs.