(may be repetitive since I did not read the whole thread)

The contents of the carry-on matter a lot!

Even the airline's brochures say to not have valuables in checked baggage. (The nature of the items has been described earlier in this thread.)

When the passenger follows the rules and is still unable to be accommodated, baggage and all, in a decent and proper fashion on the flight, the airline still has the obligation to honor the ticket with due dispatch.

Typically I suggest that someone unexpectedly told to gate check a carry-on item stop right there and quietly remove valuable objects from it and hand carry said valuables.

Travel tips: http://www.cockam.com/travel.htm