Not so much for the faith but the lost. By insisting on a young earth interpretation Christians make it very difficult for the lost who are scientists and those in fields where the proof of an ancient universe are discussed and handled daily. How can we ask an astronomer who gazes at star light which is billions of year old to believe in a universe less than 10000 years in age? How about the geology student? The palaeontologist? Why hold on proudly to an interpretation that no scripture actually demands?
The theories supported by Young Earth Creationists are clearly so absurd and false to any reasonable person, yet are indeed largely backed by the bible. This obvious contradiction causes many Christians to question not just the story of Genesis, but ultimately the veracity of the Bible itself. This eventually leads many people away from their faith.
I do not believe ideas taught by Young Earth Creationists pulls some people away from their faith, for the most part. The only people it will adversely affect are those that accept the scientific facts that the Earth is much older than this theory states. If a Christian believes in the Bible and wants to believe it word for word then these theories won't affect them.
As practical science becomes easier to understand and prove, the idea of creationism is slowly losing support. By demanding blind faith in the shadow of hard evidence kids can understand, the entire religious experience becomes questioned. Kids lose their faith because when they ask questions, they are told to believe what is told to them even though the opposition doesn't just tell them, it shows them and teaches them how to prove the theories. Creationism offers no such proof aside from obscure interpretations of a few bible passages. Kids know what a lie looks like.
Hard lines firmyl entrench people who have the belief firm in their minds already, but often can turn away people on the fence who may not be sure where they lie on the issue. People who might believe God created the earth might reject the idea if its tied up in the idea the earth is 7,000 years old.
No, I do not believe that the ideas taught by Young Earth Creationists pull some people away from their faith, because most of the people who are Young Earth Creationists are very religious. Young Earth Creationists are not incompatible with a deep faith in religion. It is the nonreligious that have a hard time understanding it.