It certainly does.
In the situation of someone getting off and someone getting on, if the person gets on before everyone has got off then their aim is met. Meanwhile the people getting off, as has been mentioned here, are at risk of being taken on to the next stop. Therefore it seems good manners that the person boarding makes it as easy as possible to allow those who want to leave to do so. And if this means stepping out of the way, or even back onto the platform, that is what is needed. Even if they step back onto the platform, the driver/guard will have seen them and will not allow the train to leave whilst someone is trying to get on.
You don't get the delays you suggest, because the train company will have recognised they are serving customers with a range of different needs, so the 'wait' after the doors have opened will have been designed to allow for seated passengers to get up and leave the train.
Of course this means the train drivers also have to follow their training and not try and cut short the 'wait', as doing so would obviously impact on the customers on board.