maniacbug:
Backwards compat is critical for people who want to re-use shields of course, but personally I have no use for it.
The Arduino (maybe Wiring?) guys didn't seem to plan much for moving beyond the 8/168/328. That may not have ever been their intention, which is fine. This whole thing probably grew way beyond their expectations anyway, and you could say the blame lies more on Atmel for not sticking to a regular and defined arrangement. Whatever, it is what it is, and given the difference between the various families, it's pretty understandable that you can't always just swap out hardware.
IMO, shield and pin mapping compatibility involves possibly more compromises than it's worth. I feel that if you're moving your project from one chip to another, a little work to change some DEFINEs or some IFDEFs in the library are perfectly acceptable. Abstraction comes at a cost, and sometimes it's just easier to cut ties.
That said, the variants out there do a great job of allowing the user to choose the layout that makes the most sense given their application. Between the various community 1284 designs, you get some very reasonable options for whatever degree of backward compatibility matters to you. That might actually be better than a single official 1284 design that chooses one layout or another as the bona-fide standard.