Yes of course God should be a part of The Pledge of Allegiance. The United States of America was built on the premise of freedom of religion. Even though the nation has grown, we should still focus on the principles that founded this nation. As long as the Articles of Independence exist, then God should be a part of the Allegiance.
America is the greatest nation on Earth, and we didn't get there for no reason. We became the greatest nation on Earth because of our faith in God, something our culture is forgetting.
Contrary to what many have said, "Under God" is not unconstitutional. The constitution says that there shall be no state sponsored religion, not that we cannot even mention God in public.
There are references to God, our Creator, within our nation's constitution and the Declaration of Independence. Not to mention, public officials swear into office on the Bible and there are other references to God in both our federal and state governments, for example the Ten Commandments in the state of Oklahoma.