The reason they say this is because of the "Earth is 6K Years Old" types out there who see scientific facts in front of their eyes(the light we see today from the Andromeda Galaxy has taken 2.3 million years to get here) and just poo-poo this fact as meaningless. THAT is the concern. When scientific facts become meaningless then science IS irrelavant. Talk to some of the 6K'ers and you would realize that this is a legit concern.
Actually, I run into 6K er's all the time at church. They know I'm comfortable not buying into a literally time analysis of the bible. But I would trust the average 6Ker to babysit my Little GoBucks LONG BEFORE I would trust Dr. So and So bio scientist at my local university...
Your average 6Ker CARES about what God thinks about the quality of that babysitting. Your average bioscientist doesn't care to think about God, or babysitting, or anything else that is 'boring' ... and thus the neglect of Little GB would be my biggest fear.
Another way to say this is as follows: if science could be shown to make people desire to be 'good', science would have far more credibility than it does now. This is why 6k'ers have traction in today's 'modern' society.
This "6K question" bothers me too. I have obtained a lot of radiocarbon dates that are older than 6,000 years, with some up to 11,0000 years. I have good evidence of human cultural activity, but no flood. Yet I am told everything I have found is wrong because "that's not what I believe." My facts and theories about those facts are dismissed.
Other scientists have a good mtDNA record of human dispersal from Africa going back some 70,000 years.
Given this type of information, why should the Young Earth theory be taught to students in public school science classes?