Then Christians would have no grounds to believe in any part of the Bible. If one part is deemed 'incorrect' or 'untrustworthy' then it is impossible to then believe fully in the rest. Besides, why then would Genesis go through extensive genealogies, numbering years if it were not true. Why bother spending so much time writing false information? If Christians cannot believe in the creation story, any other belief from a biblical point holds no value.
Yes, creationism is important to Christianity, because it is a way of explaining the events of the Bible. Some people believe that evolution is consistent with the Bible, but most Christians believe that creationism is a way of explaining how God made the world. It is not the way to get to heaven, so it is not the most important thing, but it is still important.
The extreme limitation of the creationism timeline makes it very difficult for even liberal Christians to accept. It is the basis, however, of some more conservative Christian beliefs and, in that regard, is important to Christianity. Matters of church and state are separated in the United States, though, which means that creationism has no place is publicly funded schools.
There are many Christians who do not believe in the biblical creation story or at least don't take it literally. Although Christians believe it was divinely inspired, the bible was written by fallible men and assembled centuries after Christ's death as a reference. The most important aspect of Christianity is the teachings of Jesus Christ, not literal interpretation of various bible stories.
It has no place being taught in schools because it is unsupported by facts. Teaching christian creationism in schools would be biased to other religions because it would be impossible for schools to teach the creation story from every religion. Besides, when it comes down to it, schools are there to educate children with facts. Facts are supported by verifiable evidence, creationism is not.
The creation story was concocted by people as a form of social control, a spiritual heroin for the masses... Using the fear of damnation to enforce their will on the populace and too extort money from people that think it will take them a step closer to heaven!!! Rubbish, things don't just ping into being that has been proved by evolution!