Their purpose was NOT to die. Their purpose was to guarantee that the U.S. would respond to any aggression in those areas. In the case of western Europe, it worked! In the case of Korea, it has worked to this point.
I agree with Bush that troops should be removed from western Europe since the eastern block does not exist any more. However, I am not convinced about the troops in Korea.
The existence of the troops not only provides deterrence against aggression, it also forces the hand of the current U.S. President should aggression occur. Imagine Korea being invaded from the north with Jimmy Carter in the white house and no American troops on the ground. Would you be willing to bet that Carter would have responded appropriately? Even if he had, would you want the North Korean's guessing that he might not and therefore start something? It's not a stretch to conclude that those troops may have preserved South Korea while Carter was at the helm. Republicans with guts may not hold the white house until the time that South Korea is safe from the north.
Your point about forcing the hand of the then current president to take action is a very interesting one, and it's one that I don't remember seeing in previous discussions. While I have confidence in the current administration, it's certainly worthwhile to remember that if (god forbid) we get Kerry or another Clinton in the Oval Office, we might just get a "please don't hurt us again" response from the White House to a North Korean incursion below the DMZ...