The real issue of the discussion you linked to was truth, not usefulness. There is no evidence that the tests give accurate portrayals of the test takers’ actual personalities, and so there is no evidence that the tests are useful as tools to find out the truth about the test takers’ personalities. Even thought the second part uses the word “useful,” the issue is still truth. People can find personal value in just about anything. If they value the truth, however, they should understand that there is no evidence that personality tests provide any truth at all.
Personal beliefs that the tests yield accurate results are not just irrelevant, they are part of the problem. These tests accentuate the positive, and human beings have all sorts of internal biases that make them subject to flattery—especially vague flattery that could apply to almost anyone. Michael Shermer did an experiment on “psychic readings” where he gave everyone the same opening statement and then went on to do a more “personalized” reading. All of his subjects rated him as accurate—despite the fact that he spent only 24 hours preparing for what he knew to be a fraudulent exercise—even if a few did not find anything he said surprising.
The opening statement gambit is the crown jewel of this experiment because it was exactly the same for everyone and received the most consistently positive response. Every single subject, for instance, found the last line to describe them perfectly. Why? Because it was pure flattery gold. Remember: the opening statement was generated in advance and with no reference to anything about the actual people who were profiled—except for the likelihood that they would be subject to certain biases. Yet it worked perfectly.
So what does this mean? First, I don’t think it means that people who believe in tarot or astrology are idiots. I’m not sure anyone is saying that; and at the very least, I would not assert such a thing. What I am saying is that people who believe that personality tests, astrology, or tarot give them reliable and accurate information are factually mistaken. The adverse impact of this is that it leads to believing in untrue and unjustified things. It lowers one’s epistemic standing.
A random personality description might be helpful for personal growth, but it might also inhibit that growth. This makes it risky in a way that we can’t acknowledge if we insist on saying “these tests are meaningful to me, so I’m not going to question them.” If you think that living in the real world is valuable, then, you’ll stay away from these fancy tools for self-deception—or you will at least take them for what they are and not pretend that they are anything more. They may be fun, and they may be revealing when taken for what they are; but they are no substitute for actual self-reflection and self-analysis.