Entering CC info is last thing user does and that's the step you don't want to mess up - ever! This is one scenario where you want to stick to boring, well-accepted, well-known, highly familier UX and avoid performing fancy tricks because there could always be some case where it doesn't work as expected. For example, I tried to enter random numbers in this UX and I did not got any response that numbers were bad. Even worse, it did not enabled MM/DD. For some unknown CC this would mean loss of customer that you worked so hard to compete and ultimately acquire.