I would say that the Declaration of Indepence as a whole is not very important any more. However, there is one part of it that is very important. This is the part that says
We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty and the pursuit of Happiness. — That to secure these rights, Governments are instituted among Men, deriving their just powers from the consent of the governed, — That whenever any Form of Government becomes destructive of these ends, it is the Right of the People to alter or to abolish it, and to institute new Government...
This is important because it states the foundation of the United States' political system.
Although the Declaration does not itself do anything to ensure that we have these rights or that our government gets its power from the consent of the governed, it is very important that the Declaration says these things. Because it does say these things, we have a clear statement of what it is that our country stands for.
So the Declaration (or that part of it) remains important as a statement of the core beliefs that we hold as a nation concerning where government gets its power and what it may or may not do with that power.