They are valid, as you don't always* have the check in time clearly stated on the timetable or the website, but common sense tells you not to get there a few minutes beforehand.
Let's agree that "common sense" tells you that you should get to the platform 4 minutes before advertised departure time and it's not necessary to be there any earlier than that
This is taking into account that the doors
might be closed 2 minutes early.
Therefore, we are agreeing that arriving 2 minutes before door closing time is ok.
Therefore we are agreeing that arriving more than 2 minutes before door closing time is earlier than necessary.
If you turn up 4 minutes before the advertised departure time, and it turns out that the doors are closed 15 seconds before departure, your "common sense" rule means that you're there 1:45 earlier than we've agreed is necessary.
Therefore, your "common sense" rule by definition means that people have to turn up earlier than necessary for many trains.
Which is why advertising the doors-closed time is more logical, would waste less of people's time, and would have no deleterious effect on train punctuality. So why not adopt that system?