It's what St. Augustine found to be the greatest harm to our faith:
Often, a non-Christian knows something about the earth, the heavens, and the other parts of the world, about the motions and orbits of the stars and even their sizes and distances, … and this knowledge he holds with certainty from reason and experience. It is thus offensive and disgraceful for an unbeliever to hear a Christian talk nonsense about such things, claiming that what he is saying is based in Scripture. We should do all we can to avoid such an embarrassing situation, which people see as ignorance in the Christian and laugh to scorn.
The shame is not so much that an ignorant person is laughed at, but rather that people outside the faith believe that we hold such opinions, and thus our teachings are rejected as ignorant and unlearned. If they find a Christian mistaken in a subject that they know well and hear him maintaining his foolish opinions as based on our teachings, how are they going to believe these teachings in matters concerning the resurrection of the dead, the hope of eternal life, and the kingdom of heaven, when they think these teachings are filled with fallacies about facts which they have learnt from experience and reason.
Reckless and presumptuous expounders of Scripture bring about much harm when they are caught in their mischievous false opinions by those not bound by our sacred texts. And even more so when they then try to defend their rash and obviously untrue statements by quoting a shower of words from Scripture and even recite from memory passages which they think will support their case ‘without understanding either what they are saying or what they assert with such assurance.’ (1 Timothy 1:7)
St. Augustine The Literal Meaning of Genesis
So long as YE creationists don't assert that their new doctrine is essential to Christian belief, it does no harm at all. But if it pushes away people who might have otherwise come to Him, that is a grave error and a cause for concern among all Christians.