The 478 was a marine unit. The 640 is the replacement for it, and for marine functionality, it is pretty much a feature match.
I would be satisfied with a 640 that supported XM WX in road mode. I would think that this would not be too hard for Garmin to turn up with a BIOS update.
They are already selling the 640 to their target marine market. This would increase sales of an existing product, while satistying the market at the same time.
AMA Lifer, BRC, NMA
2008 KTM 530 EXC-R
2008 KTM 200 XC-W
2006 BMW 1200GS Adventure