There are a couple of factors involved to make the F3 stand a chance of landing in a real match, apart from a well timed knockdown.
People don't seem to understand that landing a B12 on hit in the neutral is a good thing, as you are guaranteed another B12 and even better, a B21, which is Mid then low, that can't be avoided. So the correct block is always low after B12, so that scenario gives a F3 a better chance of landing, and in theory, a better chance than F4 will ever have of landing as opponents should actually never be stand blocking on first call after anything Mileena hits you with.
Because Mileena's 21 and B21, the opponent spends more time first reacting with a low block instead of standing block, because standing will open you up.
When you re-stand the opponent in the corner with B121+3 DD2, the same scenario is presented, and remember that you are playing against a human, not a machine.