I was just having a conversation about this with an architect friend of mine yesterday as well. She said that an artist's impression is a much more powerful tool with sharing design concepts because it allows people to fill in the details with their imagination. With a fully rendered drawing you don't provide any room for the imagination and it just becomes a plain representation of an architectural drawing. I think the same goes for trying to make a scaled down version of a physical model - something that also seems to be skipped these days.