It degrades the UX. For example, you'll have this situation:
"Man, this SD card is slow. I can move the apps from the SD card to the internal flash memory, but I don't have enough space. How about I upgrade my SD card? But there's 10 GB of apps and content on here. So I have to sync my apps back to iTunes, get a new SD card, and sync back. But how do I tell iTunes to sync back the apps only to the new SD card and not the internal flash memory? Some of these apps aren't using iCloud so how do I get back my user data without doing a restore from backup? But wait, how do backups work with this new SD card feature? What if I lost my iPad, got a new iPad but I don't have any SD card in there. Would it still restore?"