I tend to cringe somewhat when I hear my crew telling the boarding passengers that they need to see the boarding cards for 'security reasons'. Because no terrorist would ever be able to afford a genuine plane ticket.
Unless we're cross-checking every boarding card against passports, there's no security against stolen/falsified boarding cards either.
But checking boarding cards isn't something we do for fun. Really.
We're just trying to make sure the passengers are getting on the right aircraft. We don't care what seat you're going to, even though we might smile nicely and tell you you're heading to 24G. It might seem unlikely, given all the previous checks, but I still see passengers trying to board wrong aircraft, or getting on/off at transit stops when it's not their desired travel plans. It would be great if there was a system in place infallible enough not to require so many checks (and it wouldn't be rocket science to come up with one), but until then, being required to show a piece of paper before getting onto the plane is what is required.
If you think it's tiresome having to show a boarding card 3-4 times before going on a flight, imagine what it's like explaining why it's needed 300 times.
"Just making sure you're on the right flight" seems to be the explanation that elicits the fastest and most cooperative response from passengers.