1. By presenting pseudoscience as evidence for christianity, you drive away many unbelievers. As christians, it is necessary we evangelize, and if you evangelize using scientifically absurd claims, your evangelism will fail on anyone who is literate enough to understand your claims.
For instance, if an atheist/agnostic comes to read your paper on "How the entire fossil record was laid down by one global flood!" they will likely be turned off of your beliefs and Jesus as well. As a christian, I try to avoid confirmation bias. If I see an argument for the truth of the Bible, I do not immediately accept the argument as valid simply because it conforms to my worldview. I examine the argument, and I check the science, history, and logic (assuming it contains one or all of these things). If the argument fails, I discard it. If the argument works, I accept it as another weapon in my arsenal of evangelism.
2. It causes believers to fall away from the faith. When many christians search for scientific and historical evidence for the bible on the internet and find young-earth creationist websites like "Answers In Genesis", which are the dominant christian presence on the internet, they will find these websites and think these arguments are the best christians have to offer.
When reading nothing but terrible pseudoscientific arguments from young-earth creationists, one has little to no reasons to believe, and thus, a reason to turn away from the faith. In fact, many of my arguments in favor of christianity would fall apart if you assume young-earth creationism. As rejecting modern science creates so many gaps in our collective knowledge that many premises in arguments for christianity become uncertain. It’s a testament to the failure of young-earth creationism that assuming their claims to be true destroys most arguments for the validity of the bible.
On a side note, this can also happen when young-earth creationists say things like "If you believe the overwhelming scientific consensus on the age of the earth, you're not a true christian!" This can cause some christians to believe young earth creationism is required by christianity, and many would turn away from the faith for this reason.
3. It causes unbelievers to equate young-earth creationism with the bible. I cannot count the times I’ve defended the bible against an atheist, and they've said something like "The bible says the earth is 6000 years old and that there was a flood that covered the entire surface of the planet! How can you believe that?" So when atheists only run into young-earth creationists, they believe young-earth creationism is actually what the bible teaches, with no other possible ways to take it.
So, I usually then end up wasting a bunch of keystrokes just debunking the idea that the bible teaches a 6000 year old earth or a global flood. If young-earth creationists would not make such absurd claims or at the very least, be intellectually honest by saying that young-earth creationism isn't the only view on the biblical text, than these assertions by unbelievers would not exist.
4. It’s terrible science and terrible theology. I feel all humans have the duty to combat ignorance where they run into it. This is why we have schools and encyclopedias. This is why if you come across someone who believes the earth is flat, you probably attempt to convince them otherwise. Young-earth creationists are ignorant of the bibles true messages and ignorant of modern science.
For these reasons and more, I find young-earth evangelists often do more harm to the faith then good. This being said, the reason most of my posts so far have been on young-earth creationism is because my blogging career is in its infancy and I need a warm up. Many posts addressing unbeliever arguments and other matters will come in the future.