Is it really that hard to have your boarding card ready?
It is a CAA requirement (I believe) so all UK airlines certainly do it. It does highlight instances, albeit rare, when someone attempts to board the wrong flight.
Directing people to seats is not primarily why the cards are checked but contrary to the OP's views, directing pax to seats actually speeds up boarding, particularly on wide bodies, but even on narrow bodies some people just don't have a clue.
I'm just bemused that you are put out by having to show your boarding pass one final time but don't mind putting in the effort to post and complain about having to do so, on the Internet.