You show a good example. Would you still think that this model is better if:
1) The post man can read your mail without you noticing (e.g. the envelope is never damaged)
2) You have to provide your postman with a key to open your locker? The key might additionally fit into your other lockers (e.g. A lot of people reuse their passwords)
3) The postman can easily store copies of all the letters you receive without you knowing
4) The postman travels from your local post office, to a completely different country, where the correspondence may be inspected by the other government at will without you even knowing and then gives you the letter.
Once you consider these points, you might not like that approach unless you completely trust your postman.