"This time I took the issue up with the handling agent, who was appropriately sympathetic, but who commented that it's not worth putting a padlock on hold luggage, because "they'll get in anyway"."
I agree, unfortunately. I don't check anything valuable. I just don't lock luggage.
As far as theft is concerned, I think the airline baggage handlers are part of that too. At Washington Dulles, they have caught baggage handlers with sets of TSA-lock master keys.