and they are obvious comments on depression as they develop (solvable, many will be happy to hear, but a combination of happy thoughts, magic and chocolate).

I think that is why Lupin had to be there in the compartment. The dementors would have been too scary otherwise. It would not have been enough to have a teacher simply coming by and stopping it from the outside. You had to know you were with a teacher from the beginning of the scary.

I do not really see the point of stationing them around the school, as Black has already shown his ability to slip past them (times 100 when he gets into the school later on), but I am sure Fudge's fuzzy logic had a hand in that. It is also interesting to see that while Dumbledore says that he will not have a dementor inside the school, one does show up later on to administer the kiss (presumably), and certainly in book 4.