Anansis - I used to take a similiar line to you, until I found an instance of an inbound aircraft arriving 45 mins early, boarding beginning early as well, and boarding being closed 30 mins early - presumably so the dispatcher, crew and everyone else could get home early as well that evening.
Alas I found myself on the wrong side of a locked gate with nobody around to whom I could plead, had to pay up to have my booking changed at an office with a grumpy handling agent, as well as find a hotel room at 9 pm. Somebody was probably breaking formal procedures, but there was nothing that I could do about it.
Strongly recommended to consult an arrivals monitor before deiciding the Last call message is a case of someone crying wolf !