The reason why I favor teaching evolution in schools -- rather than creationism, ID or whatever other guise the anti-evolution movement chooses to masquerade its "alternative viewpoint" under -- is because evolution is a theory, while creationism / ID / whatever is not.
You don't need to subscribe to any particular belief system to understand this. The reason has nothing to do with faith or religious feelings. It has to do only with the definition of what a theory is.
In order to qualify as a theory in science, an idea must be able to cover two things. First, it must explain something. Both ID and evolution do this when they address the question as to how life got to be the way it is today. In this respect, they both hold more or less equal merit -- the direction Occam's Razor cuts depends on in which direction you perceive the "simplest" solution to lie. I don't think any one side's claim to have the most "rational" solution holds any more water than the other ... and while evolutionists claim that circumstantial evidence backs them up, creationists can consistenly (within the framework of their idea) claim that the illusion of such evidence follows just as logically from their idea that there is a creator who made it that way.
Where creationism fails is in the second test of a scientific theory -- it must be predictive. Evolution is a predictive theory; creationism, by its very nature, cannot be.
Using evolution as my basis for understanding the changing nature of life (contrary to popular opinion, evolution does not address the origin of life!) I can make a prediction that, under a certain set of circumstances, I will see a certain set of results in a population of critters in my lab. I can predict that if I sterilize every male and female with Trait X and only allow those critters with Trait Y to breed that, in a few generations, I will see a greater incidence of Trait Y in my population than Trait X, compared with the population I started with. I may further predict that, even if I stop sterilizing critters after a few generations, Trait Y may continue to outpace Trait X due simply to the sheer number of critters that have it over those that have Trait X. I can then devise an experiment to test whether or not my understanding of the situation is correct. If my experiment bears out my prediction (and I did my experiment right) I can mark a tick on my belt for the Theory of Evolution -- it got another one right!
Creationism, on the other hand, cannot make such predictions. It gives me nothing to go on if I want to try and modify animal populations. Other than giving me the warm squishy feeling that I have some understanding of the origin of things ... I gain no scientific insight from creationism (religious insight is another matter, and not one I'd like to get into ;)
So ... which of these viewpoints would you rather your budding scientists learn in school? By understanding evolution, understanding why evolutionists have come to believe the things they do, how science tests theories like evolution, and what makes evolution a real theory, and not just a collection of dogmatic decrees, children learn to understand science as part of a bigger curriculum.
And evolution is an extremely successful theory in modern biology. Excluding it from the curriculum would be like excluding Newtonian mechanics from physics classes; excluding DNA theory from genetics; removing the periodic table from chemistry. If you remove the religious trappings from the scientific body of knowledge that is evolution, you find a theory that not only neatly and accurately explains a wide range of phenomenon in the biological world, but that also make predictions about how that world might change under a controlled set of circumstances. And that is the tonic that gives modern science its power.
So, religious arguments notwithstanding, I really don't see how there can be any debate about whether creationism is a theory, same as evolution. It just isn't. Is it appropriate that ideas that aren't established theories be taught in public schools? IMO, not really. That's what home schooling and private schooling is for. None of us wants our tax dollars wasted on teaching just any wild idea that floats across some school board rep's giddy head ... and whether you think creationism is one of these or not, you'll probably still want to draw the line somewhere. If you drop the predictive requirement on acceptable theories to allow creationism into your curriculum, you will also soon be seeing other, less savory ideas creep into the fold under the same guise -- and you won't be able to keep them out without coming off as a total hypocrite.
If you want to believe in creationism, fine. If you want to believe in evolution, fine. But keep your biases to yourself, and teach kids science. Good science. Real science. Then we'll see who is right in a hundred years or so, once God or natural selection has taken its course.