I think that the symlinks are there just for backwards compability issues with apps, that look under /sdcard/ mount point. it doesn't have anything to do with fs, android never used fat, or ntfs; known fs in android are yaffs2, ext4 and maybe a few proprietary ones like sammy's rfs.
The real sdcards were formatted as fat, and most apps that used them to store data only knew how to read/write a fat fs. I found this out by creating a mountable volume on a real sdcard on my old phone. It mounted as a directory on the sdcard. If I formatted the volume as fat, all my apps could use it normally. If I formatted it as ext(anything), a few apps could use it but most could not.
The symlinks are for backwards compatibility, but I believe the fuse filesystem virtual sdcard0 exists to make ext4 look like fat to all apps and to windows.
Note that if the gnex "sdcard" had its own partition, it could be formatted as fat and mounted as USB external storage like a real sdcard. I don't know, but it seems like Google went out of their way to prevent that.
The Google line is that the gnex sdcard shares an ext4 partition to maximize space, ie no unused wasted space on two partitions. But that is pretty farfetched, since the number of problems caused by this arrangement is huge in proportion to the "space saved" which is negligible.
Obviously, the real reason for this kludged-up mess is to discourage local storage/backup and encourage cloud storage, which can then be monetized.
. But I guess the app devs figured sdcards would be fat forever. so they only included fat I/O in their apps.
