I'm rather horrified that people on here thinks that us cc checking boarding cards at the door has no impact on the safety of a flight. Imagine this: some "unstable" person books in for a flight with checked in luggage and blatantly walks onto another aircraft. There's explosives (or even something as small as a box of "Lucky Luke" matches in this person's luggage. The "unstable" one travels to ABC safely, but the luggage travels half-way to DEF before bringing the aircraft down. Therefore, it's a security risk.
If all cc could explain to pax when they ask, that "I'm just checking your flight number and date", pax would understand much better. On a short haul aircraft (single aisle), there is no necessety of telling people where they're sitting (unless they ask).
I try to do the boarding card scenario as good humoured as possible, by sometimes saying that we don't want any uninvited guests on board, or just checking they're goiog to the right place. All with a smile of course. It helps the pax understand and then accept why we do it. Trust me, we don't do it because we desperately want to, it's what we have to do.
Hope this clarifies some issues.
Gg