I encountered the request for sight of boarding cards by the guy on the conveyor for the first time at the weekend. I asked him why they were needed (twice - the second time employing my usual charm and tact) and he told me it was because of people jumping over the barrier in frustration with the scanning/photo taking process.
I've had problems with these machines since they were first installed. There are a couple of gates which won't let me through, telling me to "seek assistance". I've found that moving to an adjacent gate usually (but not always) works. These things it seems don't actually do a biometric scan - it's always a photograph that appears on the screens at the second check down in the pier alongside a big red rectangle containing a message along the lines of "unable to carry out biometric reconciliation - check photo" or some such. Or perhaps they try to scan but fail and revert to the photograph. I must be shifty eyed or something to upset them so often.
They have the same system at LCY (same ownership) but without the biometric scan/photo process because LCY doesn't let inbound and outbound pax mix or allow transfers airside. Personally I'd be happy to avoid the chaos that is the main departure area and be routed to a domestic flight only lounge, and no longer have to endure this process. But only if there was a Pret or similar in it of course...