I am no great fan of T5, its layout is appalling. But the conformance idea makes sense to me.
Too many times have I been on a plane that has been delayed whilst they try and track down the last passenger who is missing and then offload his luggage.
Who should suffer - the person who is late or the hundreds who made it on time?
As for discretion - in this case it is better to have a clear rule that everyone understands.