Not to mention that IQ tests mostly measure your ability to perform well on IQ tests. Which is at least somewhat correlated with practising these tests.
by someone

 There is a bit more to be said for them than you suggest here (which doesn't mean to say that I'm a hard-line advocate of IQ tests). I seem to recall reading that some research had shown that there was little gain after about three practice sessions - maybe there's some more recent research. In general, cf. a crtic of them:

Harvard professor and paleontologist Stephen Jay Gould argued that intelligence tests were based on faulty assumptions and showed their history of being used as the basis for scientific racism, although did not at any point attempt to scientifically refute intelligence tests.
...

Gould did not dispute the stability of test scores, nor the fact that they predict certain forms of achievement. He did argue, however, that to base a concept of intelligence on these test scores alone is to ignore many important aspects of mental ability.

http://en.wikipedia.org/wiki/Intelligence_quotient#Criticism_and_views

I think most people would agree with the latter point, including those advocating IQ tests.

Cf.:

In response to the controversy surrounding The Bell Curve, the American Psychological Association's Board of Scientific Affairs established a task force in 1995 to write a consensus statement on the state of intelligence research which could be used by all sides as a basis for discussion.
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In this paper the representatives of the association regret that IQ-related works are frequently written with a view to their political consequences
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The task force concluded that IQ scores do have high predictive validity for individual differences in school achievement. They confirm the predictive validity of IQ for adult occupational status, even when variables such as education and family background have been statistically controlled. They agree that individual differences in intelligence are substantially influenced by genetics and that both genes and environment, in complex interplay, are essential to the development of intellectual competence.

ibid.