I live in Northern Virginia, where we have two area codes, 703 and 571 as an overlay to 703. The D.C. metropolitan area uses 202 (D.C. proper), 301 with 240 as an overlay for D.C.'s Maryland suburbs and 410 for Baltimore. In the entire metropolitan area one has to use all ten digits to dial a number, even if it is in the same area code as the originating phone. Thus the notion of an area code for distinguishing between local and long distance calls is fast becoming obsolete, which finds its expression in most people showing their numbers using the dashed notation (xxx-xxx-xxxx), not the one with the area code in parentheses [(xxx) xxx-xxxx]. Without trying to offend people in less densely populated areas, a seven-digit phone number seems quaint to me now.
Apple's software engineers probably all live in the greater San Francisco are, where there aren't any area code overlays, which would potentially explain why they didn't think to allow xxx-xxx-xxxx as an alternative format. There are in the L.A. area, however.
When I did the Google search that led me to stumble upon this discussion, I was hoping to find a defaults write mechanism for changing the preferred format, but I haven't.