I can only post 4 images in a response, so I am dividing this response into a second part.
And I am starting with this image of Roddick serving at the point of contact, with his body turned so his arm sticks out from his side, not with his arm reaching up.
Todd Ellenbecker in the video that Chas has posted above ( Rotator cuff injury (20:25 ) http://www.tennisresources.com/index...2&ATT=&reso=hi
points out there is something "magical" about the arm being extended out from the body at 90 degrees.
There is much more power that can be transmitted from our arm musculature at this angle.
That is why "overhand" major league pitchers keep their arm (shoulder) angle this low.
Same for football quarterbacks: