Purpose of boarding card 'check' at aircraft door

[flyblue]

Yes, mistakes can and do happen, and it is not advertised in the front page of the Times when it happens. Which means that even the most experienced pax might not be aware of the entity of the problem.
Not all airports have the same degree of security (otherwise we wouldn't find now and then some poor guy frozen in the landing gear compartment). But even in the best and biggest airports mistakes happen. A few years ago at my airline the topic of the Security annual course was exactly the BP check at the door, with numbers and percentages. And boy, do we (aviation industry) still need that check, and for various reasons, which it would be too long to enumerate here.
The head count alone is not enough. Think of a station in some third world country, where one pax who should board gets to not board, and another one that shouldn't does. The head count would be right, still the situation would not. We have some stations where the head count is not mandatory any more (stations that have been labeled "secure" by a Quality check) but still the BP check is mandatory. You have to multiply the checks at each stage to make sure you have closed all the holes in the Swiss cheese (sorry TightSlot, I know this one will make you wince ), because it's not only one hole you're covering with one check, but many (as many as the things that could go wrong at each stage).
I know that from the outside it is quite difficult to understand all the reasons for a rule, because without knowing the procedures and all of its stages (where something could potentially go wrong) you wouldn't see the reasons behind it.
Without the BP check, a few months ago on one of my flights mother and son going to YUL would have ended up in CDG instead And don't think that "they would have found out eventually". One of the examples we analysed on that Security course was of a pax that having boarded the wrong plane and heard the announcement that the flight was to XXX, thought it was a joke and didn't say anything until it was too late. Remember: at each stage something can go wrong. An incident/accident happens when a string of things go wrong. Try to reduce the number of things that can go wrong ( = multiply the checks) and your chain of events is broken.