The debate over the existence, and mutability, of IQ is a tricky debate. I am a follower of the debate and try to keep a very open mind. But as I am not a statistician or a psychometrician, I feel that the best I can do is to read and decide on conclusions made by others. And as I try to keep an open mind, I often feel like my opinion and inclination changes with each new book or article that I read. (I try to remind myself that this open-mindedness may be a good thing in a debate so doctrinaire.)
One thing I can't help but think about, though, is that like so many other debates, this one often seems vulnerable to what I call reverse reasoning: figuring out the conclusion you want to come to and finding a way to get there. In particular, any debate with social ramifications for action are particularly vulnerable to this type of reverse reasoning.
In the case of the IQ debate, concluding that IQ is relatively fixed can be used to justify some sort of passivity on the part of educators and policy makers. Libertarians are likely to like this option because it makes arguments for increased education spending (like Head Start) a bit harder to make. This argument is also likely to appeal to those teachers who are frustrated and exasperated by working with students and being met with little to no demonstrable improvement. (I've been here and I can attest that the frustration felt when working with underachieving kids meet with few real results certainly make this view look a bit more appealing. That way, teachers can take the blame off of themselves and place it onto the students' fixed limitations.) I don't think teachers consciously do this, but often this view of a fixed intelligence will seem more appealing to those frustrated by hitting a ceiling with students.
On the other hand, the view that intelligence is a fluid, rather than fixed, entity is equally vulnerable to reverse reasoning based on where one wants to end up. I find that this view is almost always that chosen by those with strong egalitarian political beliefs, and strong aversions to suggestions that some are better than others "by nature." This is also the view that predominates the education field. My guess is because seeing intelligence as malleable provides a good rationale for the field of education. It is much easier to be an educator when one chooses a view of intelligence that sees education as integral to its development. (This is like the idea that it is much easier to play the lotto if one has a belief that one can win.)
The IQ debate is so frought with emotion that I worry about the prevelance of reverse reasoning in the debate. I worry that many of the people I've talked to have made up their minds more based on what they want to see happen than on what facts suggest. I try my best not to take sides on anything other than the facts of the matter (and the reason i've not really taken sides is because the facts of the matter seem quite open at this point).