The more defectives you have in a given population the more likely some are to get through the screening process. A pretty sizable chunk of the Saudi population thinks it is a great thing to go down as a martyr killing American infidels. All it takes is one to get through and here we are. For Al Qaeda and their type it is a double win having a Saudi do this: kill Americans while damaging the relationship between the Saudi and American governments.
On the subject of “defectives” because of restrictions on women, they have few opportunities to meet prospective husbands outside the extended family. Thus first and second cousin marriages, throughout fundamentalist Islamic countries, are probably a significant cause of birth defects and mental and intelligence issues.