psy tests
From my understanding, the psychological tests used in pilot selection are not a pass/fail type of deal, but and aid to the recruitment people in the selection of suitable candidates. When they have the results of the psychological test, they can compare them with the impression the candidate left at the interview stage and the answers given. If there is a discrepancy between the psychological report and the answers given in the interview, it is a possible indicator that the candidate was lying in either the test, the interview or both.
As far as the point that if a pilot is qualified, he is good enough, from my experience this is probably not true. I have met many people who either hold or are capable of holding an ATPL, but I would not personally want to spend 12 hours on a flight deck with them. People who, for one reason or another would not fit in to a particular airline have a negative impact on all those who they work with, and as such when recruiting, an airline will try as best as possible to select the candidate who will fit in. The psychological test can be an integral part of this selection.