I'm yet to meet a QF driver who refuses screening point blank. I've met a few who've asked to be screened privately or more appropriately but blanket refusal. There is more to this than meets the eye and perhaps we should wait for it to all come out in the wash.
As for the success or otherwise of screening, I support doing it but I don't believe for a second that it's fool proof or has the ability to block 'weapons' or DGs from getting on board. I know that because I've been a screener- albeit a long, long time ago. The assessment as to the cognitive skills of most (but not all) screeners is correct. A lot of screeners are that because they can't get another job elsewhere. I lasted just three months before I went hunting for a better job elsewhere. Found it and started within four days.
