The Declaration of Independence is one of the founding documents of our nation, and is frequently quoted in Supreme Court and other federal court decisions.
It certainly illustrates the intent of our Founding Fathers, and is used in that regard.
The Declaration of Independence was written and signed by the Second Continental Congress, a war appropriations body. The US Constitution was written and signed by the Confederation Congress, a legislative body. Furthermore, the US Constitution was ratified by the legislatures of all thirteen original states. The Declaration of Independence was not. The Declaration is the only positive thing that I have to say about Thomas Jefferson, but in terms of legal precedent and philosophy, it is entirely irrelevant.