"capacity" for laying is best measured from the keel bone to the pelvic bone. Plus the distance between the two pelvic bones is important for determining egg production in hens. This is the Walter Hogan method. Ideally you would imagine that to be true in regards to a broad back but I've noticed leghorns that aren't that wide have amazing pelvic widths. As for how the SOP is written, IMO; I feel that it was written with what you mentioned "to support, dependable production bird" but I also feel there is some contribution to a strong appearance.....but then again it is the standard so that should be the guidelines followed. For people depending on the birds dual purpose contributions and not exhibiting the fowl; what does it matter what the appearance is as long as they don't try to sell them as "show" fowl.