I don't think anyone answered the OP's question about the reason why they have such a dished face.
Don't take this as gospel, as this is what I'm remembering from my Equine Exercise Physiology lectures, but the dish in the face developed due to the hot, arid climate the Arab evolved in. The dish effects the shape of the sinuses, which, coupled with the large nostrils, allows a large oxygen intake and cools the air efficiently, helping prevent potentially fatal overheating of the brain.
Compare them to the draft horses/British native ponies - they often have a more roman nose. The larger sinuses allow the air to be warmed before it is taken into the lungs, as they are typically from colder climates.