Two arguments here, one side says requesting boarding cards at the door is a security precaution and the other says it's revenue protection. The first thing to say is most european airlines don't check boarding cards at the door, (though Air France is the exception)
It's probably fair to say that the majority of people who commit crimes on aircraft are in possession of a valid boarding card. Furthermore, the crimes they carried out would have been committed regardless of whether they'd had their boarding card checked or not. Also, a passenger getting on the wrong flight is not really a security issue, as Mancry mentions it sets in motion a procedure, this procedure will cost the airline money because it's their fault it happened and they will have to pay to get the passenger to the correct location.
I couldn't care less if a FA wants to see my by boarding card at the door or not, if they ask me for the card I will show it to them along with the correct courtesy. But, to suggest this is a security issue is simply incorrect, it's revenue protection and nothing else.