Religious types hate their children being taught about evolution in school science class because it clashes with their beliefs. On the one hand those beliefs are wrong. On the other hand, however true it is, evolutionary theory is neither useful knowledge or training for children and could be dropped from the curriculum without doing any harm to their education.
Why is it so important to teach the theory of evolution to school children?
1. It is true
Yes, but so are lots of things that we don't teach in school, either because they're too difficult (Einstinian physics) or we don't think they're important (like the history of other people's religions).
2. It is useful to know
No, in almost any line of work, including much of the life-sciences, knowledge of evolutionary theory will never be directly relevant (any more than driving a car requires a deep understanding of the principles of an internal combustion engine).
At the societal level, as far as biology goes, understanding germ theory and the attendant public health issues (especially vaccination) is far more important to have entrenched in our civic common sense.
3. It's important for understanding how scientific methodology works
This is only an issue if creationism is taught in the biology class as science. Otherwise students will pick up all they need to know about how science works in their studies of other areas of biology, physics, etc.
Why should we drop evolution?
1. It's a political issue
As Daniel Dennet has said, evolutionary theory “eats through just about every traditional concept, and leaves in its wake a revolutionized world-view, with most of the old landmarks still recognizable, but transformed in fundamental ways.”
Isn't it reasonable not to want one's cherished beliefs dissolved by acid? Doesn't it interfere with your ability to raise your children with your traditional values? What we teach our children is a matter of collective choice about our values - not a reading off of the truths of nature - and therefore something that political communities can and should democratically debate about and compromise on if necessary.
2. It's a culture wars thing
There's no real reason for liberals to be so invested in this obscure curricular issue apart from the culture wars. It's a shibboleth whose only significance is as an indicator that the weirdo religious people, with whom we enlightenment liberals unfortunately have to share our society, are 'winning'.
3. It's a distraction
Even within the culture wars arena there are other issues it is much more important to win. For example, mandatory sex education would be far more useful for children's lives than reading about evolutionary theory.