Well that and the fact that wifi can drop out at any point and usb cables tend not to.
That would be my biggest concern. I tend to reboot my router at least a couple times a day, I'm at the end of a long line and the internet can drop or die until you refresh.
So any long prints I'd started over wifi would be interrupted fairly regularly.
I just don't see it as being a great choice for an either or situation. And for a couple extra bucks - why not have both ?
Or failing that an sd card reader - now they really are cheap.
Just some kind of option other than wireless, that is genuinely uninterruptable.

Hell stick it as an optional extra for $20, I bet most people would opt in rather than out.