I don't now the reason other people do it that way, but if I was building a mesh I would model it with arms out because it is the median of the likely radius of motion. In other words, that position is the same number of degrees from straight up or straight down. If you modelled the arms straight down, then in a straight up pose you would have twice the amount of deformation of the mesh. If you modelled them straight up, you would have twice the deformation when posed straight down. Of course this logic begs the question "why not model with the arms rotated 45 degrees in round the y axis?".