And while shoplifting is a legitimate threat, are non-shoplifters going to be turned into shoplifters without a checkout? Are normal shoplifters stopped by checkouts? These are the core questions, and until it is tested nobody will know for sure.
Target is getting awfully close to this. With their Cartwheel app you're meant to scan all your items as you shop (so it auto-applies coupons and discounts); but they haven't taken it to the next logical step and allowed you to provide your Cartwheel output at the checkout for checking out.
I will say that the way Target has implemented smartphone barcode scanning makes me think that there might be a future in all this. It is extremely painless, they just need to stop kicking you out of the scan screen when it finds a discount (i.e. it doesn't kick you out if no discount is found, but does when a discount IS found, that's problematic for efficiency reasons).