The President should have to take the oath with "So help me God" the fact that it puts him in a binding contract with Americans belief system of God is always watching. If someone is going to take control over a whole Nation they should be held accountable by someone or have to make some vow in order to maintain their office that way they are punishable if they do not commit.
I honestly feel that the president should indeed make an oath to God when they are inducted into office. However, this process could also be re-written. If ever one day our next president doesn't adhere to the Christian faith, I believe we should change this oath to be more relative to the individual, be they Christian, Muslim, Buddhist; it shouldn't matter.
The presidents have been saying this since the first inauguration in 1789. The tradition should continue unchanged. It's also like god's existence in the world. He is unchanging and very powerful. It puts the president in a binding contract with the american's belief system of god. It should continue to be a tradition.
I'm an atheist, and although I would never get elected (aside from the fact most Americans wouldn't vote for an atheist simply because they're an atheist, I'm a terrible public speaker) I don't think that the president needs to swear upon the bible and say "so help me God" unless he wants to. While this would infuriate the devout of America, I think it would emphasize the freedom of religion- you're not favoring one religion (Christianity) above others, like Buddhism (no Gods) or Hinduism (many gods) I don't believe that a president has to put his hand on an old book and say "so help me God" to be a good leader, that would be stupid.