Wake up, thread!
After reading a few "studies" and "news" rumors getting dumped into the forum then debated fiercely, let me say that the answer to the OP question may be that "we" need to stop accepting just anyone into EMS training beyond basic first aid, medical first responder, or EMT-B.
(As for who "We" is, as of now, there is no "We" except maybe as a contentious and disorganized class of people).
How can we ever progress past "cookbook" protocols if the people entering the training for basic and advanced EMS are naive and undereducated enough in how to be informed consumers of the news and it's so-called "statistics" that, on the one hand, they will accept whatever they are told by the mediae and run with it, but on the other hand cannot recognize or believe real data as facts and then take them into consideration?
I think the more important question isn't when to bounce orientees, it's how to screen them first before you go on to having to judge/guide/teach unproductive rookies who should never have gotten to their probationary period anyways.
Sample job app questions:
1. Space aliens build the Interstate Highway system.
2. Long spineboards are a definitive treatment for any fall from any height.
3. A study is scientific if it is published in any part of JEMS magazine, regardless of the size of the sample or who conducted it.
In fact, all medical directors and supervisors should be required to take this sort of test retroactively, wearing a polygraph because we all learn to mimic and lie to keep our cushy jobs.