Then, sure you never tried a Pacific fleet in AAR. Try mixing USA fleet, UK fleet and some marines in the islands. Try moving this each turn and you'll see why you should keep marshalling cards. Still, z6 was pretty small. And let's forget when Japan goes and attack the mixed fleet.
I think the reason for removing marshalling cards and making China a popping inf country is the same: economical crisis. You know, those are less miniatures and more cards to do, and AH or WOTC need spent less money. Their legitime choice, of course. Of course, also is our legitime choice mod China and take some marshalling cards from AAR