Can't see why it's a vicious circle; more like throwing good money after bad.
A lot of students/pilots won't ever have done aptitude tests so I can't see the relevance to your point. If someone is continually failing ATPL exams, it's not because an aptitude test incorrectly predicted the outcome but because the student has not applied him/herself and become lazy.
From an employer's point of view, it's a concern as a bad track record in exams might indicate that the student would struggle to pass a type-rating paper and no employer is going to be too sympathetic.
Cheers
Whirls