The most important argument for teaching evolution in high school and middle school biology classes is one of necessity. If the purpose of biology classes is to foster scientific literacy and prepare people who want to enter the sciences--and I believe that is the purpose, a purpose which is uncontroversial in and of itself--then it is important to teach the accepted paradigm of the field.
The accepted paradigm and explanatory model in biology is the theory of evolution by natural selection. We do our students a disservice by not acquainting them with that model.
Now, I have a lot of problems with evolution, both philosophical and political.
Philosophical: it doesn't prove anything, at least not in the sense of having predictive value. In other words, I cannot agree with those who assert that "evolution is an established fact." A fact can be, in theory falsified. But I'm sympathetic to the view that evolution by natural selection is tautological--true by its logical form alone and therefore not falsifiable. If it is indeed a tautology, I'm sympathetic to the view that it is a useful tautology. (I'm plagiarizing this point from something I read as an undergraduate, but I forget the author and title.)
Political: advocacy for evolution by natural selection is often used as a club to beat non-atheists and to wage straw man arguments to "disprove" the existence of God. People who are uneasy about the supposed implications of evolution are caricatured (by those such as Christopher Hitchens and Richard Dawkins) as unenlightened bigots who stand in the way of "progress." While I don't believe that the theory of evolution by natural selection need be inconsistent with most forms of religious belief, in this day and age it should be acceptable to have those different beliefs.
Advocates for teaching evolution in schools often cite the opposition of religious conservatives. Many of these conservatives--or at least the politicians who claim to speak for them--voice the opinion that teaching of evolution in schools is an imposition of one set of religious belief ("secular humanism") by the state. These conservatives, evolution advocates a la Hitchens tend to say, are deluded and are trying to use the state to impose their views. Evolution, for these advocates, is science and truth, and those who stand in the way of truth must be defeated.
The stakes are both greater and lesser than either side would have one believe. They are greater because it's a question of providing our students with basic scientific literacy, and all this bickering comes at the expense of their knowledge. It is lesser because both sides are wont to insist the issue is an apocalyptic struggle between "religion," "truth," and "science." Advocates for the teaching of evolution by natural selection would do well to focus on social utility arguments--i.e., it's necessary for scientific literacy--and should abstain from the philosophical and political/religious arguments that merely cloud the issue. For the fact is, one can sincerely believe that the earth and all its species were created were created by a supreme being 6,000 years ago and yet still suspend their disbelief long enough to learn the underlying assumptions behind biological science. These assumptions, regardless of their truth or falsity, are used to understand nature in a specific way. By most measures, if advocates for teaching evolution simply limited themselves to the question of scientific literacy and forewent the occasional jibe at "fanatics," their arguments would carry more weight.
I do not here and now address the side issue of "creation science" and "intelligent design," except to say that I would have no objection to them being presented in a "philosophy of science" class, along with a discussion of the philosophical underpinnings of the theory of evolution by natural selection. The universe is a complicated place; no one really understands it.