Not Worthless --- Just Not Very Good

How can it worthless? It does better than astrology, palmistry, and phrenology. So it has more worth than these techniques.

Well, there are different levels of “not worthless”. Were I a personnel executive, “better than astrology, palmistry, and phrenology” would be grossly insufficient to recommend a technique.

“astrology, palmistry, and phrenology” could be of negative worth. Then graphology could be worthless but still better than them.

Well, that still doesn’t prove it is worthless.

You’d have as much luck in predicting a person’s personality by examining the wrinkles in their forehead.
Imagine you’re given a multiple choice test with 5 answers per question. Someone chooses all “A”, and gets about 15% right. Someone else chooses all “C” and gets about 17% right. I choose entirely randomly and get 20% right. My entirely random choosing did better than either of those two, but random guessing would still be a ‘worthless’ method of taking tests.