IQ tests do not assess. However, IQ tests are valid predictors of "book smarts" and excellence in domains that require "book smarts".
[WS:] I would be more specific than that and say that IQ (and scholastic "aptitude") tests are good predictors of the ability to process information like a computer - quickly and within the programmer pre-defined range of choices. They do not predict the ability to think outside the box, contemplative thinking or the so-called social intelligence i.e. the ability to engage in a dialog with others. Actually, people who are good at the latter cognitive tasks are severely penalized on IQ and scholastic aptitude tests either because they cannot score enough points within the strictly prescribed length of time, or because they do not apply cognitive frameworks assumed by the test designers, which leads them to "wrong" answers (in framework of the test designer but not in that of the test taker), or because they lack the stimulation provided by intellectual interaction with other people.
If I were to identify only one negative effect of WASP mentality on the American life it would be IQ testing - the quintessential jeder fuer sich und gott gegen alles (everyman for himself and god against all) - an assembly of individuals completely isolated form one another, and engaged in the solitary task of diligent processing of predetermined tasks to find out how they "score" and thus whether they are eligible for "salvation" i.e. admission to some institutional "heaven." Words cannot describe how much I detest this mentality. I experience genuine anxiety attacks while experiencing this attitude, even in the works of art (cf. "The Name of the Rose" or "Fanny and Alexander"). The Market with its reliance on craftiness, serendipity, and hedonistic gratification is certainly human, if not humanistic, by comparison.
Disclaimer. I did OK on IQ and scholastic "aptitude" tests. I was subjected to the former because I was a "problem kid" at school, and I had to pass the latter to be admitted to a graduate school. However, the amount of stress that I endured during this ultimate 'surveiller et punir' experience left me with profound distaste for the rest of my life.
Wojtek