Yeah, I think creationists are wrong, but what harm do they do?
Education?
Education? You mean private Christian schools? Private schools have the right to teach what they want, and should go unmolested.
Creationists have tried and continue to try to spread their ignorance into public school curriculum.
What practical harm does this cause other than the facts being wrong?
How is that not a big deal to you? It's deliberate misinformation at best... and propaganda at worst.
Would you be fine with an advocacy groups that demanded for spontaneous generation or flat Earth theory to be presented as fact in your son's biology courses? Those have as much credibility as the belief that our planet is 6,000-years-old.
"Other than the facts being wrong..."
Because the effects of the incorrect information are basically nil. Believing or not believing in evolution means diddly squat for 99% of the population, including most of the medical/scientific community. I'd put it on the same level as string theory or quantum mechanics. The engineer can disbelieve it and still build a perfectly acceptable bridge.
Contrast this with certain ethical or historical views, which have far more practical effects on students. Telling a 16 year old that there is no difference between having a baby alone & out of wedlock & having a baby while married is far more harmful than telling them the Earth is 6000 years old.