If you want an SD card that is also readable by the digital camera ecosystem, what choice do you have?

Reading external flash media becomes hard if you don't have FAT support. But agreed, anything internal should have used a sane filesystem (I know that's hard to find. At least an open one: I see ext* used internally, but FAT32 for removable flash).

You need to support it if you want to properly support removable SD cards.

Microsoft managed to make their patent encumbered exFAT file system a required part of the SDXC card spec, just as their much-abused patents on FAT32 were expiring.

I guess it is gracious of them to partially undo the damage they've done, now that they have given up on Windows Phone.