Since the power brake system is vacuum assited with a vacuum booster I don't see how a missing fuse would have any effect.
Some earlier ABS systems did not have vacuum assist, but instead a small electrical pump that was in use every time the brake pedal was depressed. On that system, it would have been unwise to have disabled it electircally as you would have had no brakes.
As for why someone would want to disconnect the ABS, some highly skilled drivers would rather deal with the possible skidding on their own, without electronic intervention, just as some would rather not have traction assist. Personally, I would rather have the ABS, but some would not.
