I was never really sure why it's an either/or, even if evolution is dead wrong, why does that make creationism useful? Or why couldn't god have 'created' evolution, the same way we now accept that stars and planets can form naturally, but god may have 'created' the Big Bang. God can keep abiogenesis until science can recreate the feat.
As to my 'aha' moment, two words, infinite regression. One of the standard arguments of creationism is that life is (irreducibly) complex, so it must have been designed. This is commonly given as the watchmaker argument, which in and of itself is wrong, but lets assume it is valid. Then who created the creator? In the watchmaker's case, god had to have designed him, and then we ask who designed god? We need deities to create deities who were themselves created by yet higher deities, ad infinium.
But people lose their jobs for teaching creationism in science class because it isn't science. Simply put, it isn't a theory as 1) it is unfalsifiable, and 2) it makes no predictive statements.
As to the 'teach the controversy' issue, even if evolution is wrong, it still is right enough to predict many things that have happened and discoveries that have been made. We still teach kids that F=ma even though Newton was wrong, and we've since come along to the general theory of relativity (which even that is starting to show problems). However, for many applications Newtonian physics is accurate enough to be useful. Thus even if evolution isn't 100% perfect (no scientific theory or law is) it'd still make a good primer for the field. The 'controversy' can (and should) be taught at a higher level and even then we should ask for a new theory that will make predictions in the field, not some hand waving 'god did it' explanation.
All of the so called 'basic flaws' I've heard people claiming to have found in evolution I've found have turned out to be like a precocious grade-school kid smugly telling their teacher "Aha, subtraction must be flawed because if I have three apples and then subtract 4 apples, then it makes no sense and is wrong" or even "Hah, you English teacher made one spelling mistake, therefore everything you've ever taught must be false".