ElleBee makes a good point, vis a vis, people not testing well etc. I am one of those people. Well, at least the essay portions. I simply cannot write well under exam conditions.
Also, if standardized tests are meant to be an equalizing factor, test prep should be explicitly disallowed, or free, whether it be books or private tutors. Otherwise, students with more resources can learn to do well on the test, giving them one more advantage.
So, yes, in the current state of affairs they do (but I am quite biased...)