If they happen to guess better the first time they take the test, their scores will be lower on retest even after many months of instruction and a great deal of learning. Tests such as the TABE test which give results in grade levels do adult learners a great disservice. Adult learners reading at, for example, a third grade level, know so much more than a typical third grader as a result of their life experiences. Testing can be very stressful for some learners and therefore may not adequately reflect what they know. This may be particularly true for oral tests.
Standardized tests lack predictive validity.
It has been shown many times that the fact that someone does well on a standardized test does not mean that they will do well in a job-training program or on the job. There are so many other factors that enter into being successful at work. Standardized tests should not assume too great an importance in determining who is selected for job training or work positions. For people who are interested in reading more about the objections to standardized tests, I recommend Elsa Auerbach's Making Meaning, Making Change for an excellent discussion. In spite of the fact that there are so many objections to standardized tests, their ease of administration and the ability to compare students' progress over time with a numerical score are very attractive features, especially to teachers who do not have a lot of time.