Originally Posted by christep
and in principle it isn't really necessary since the official IATA ticketing rules are (I believe) that a ticket is valid on any airline that flies that route unless there are endorsements that limit that validity.
Not quite -- the official IATA rules regarding endorsement requirements are so complex that they include a full-page flow-chart.
You are correct in saying that, in general, "a ticket is valid on any airline that flies that route unless there are endorsements that limit that validity", however that still does not negate the formal requirements.