This is a bit of a theoretical question I guess--and I remember Jono asked this, but can't find what his workaround for it was.
AT times I like to tweak the focus with MF assist on an AF lens. I generally AF, then twist focus ring to go into MF assist, tweak my focus and want to shoot. However--I don't like to shoot without returning to the composition--and doing that means one has to use the shutter button again--thus bringing you back to the camera's focus, not yours. One can move to MF after AF--which brings you more quickly to the point where you want to adjust focus.
Is this a better way? I did find that you if you are holding the shutter button, go to MF assist--you can hit delete button without having to release the shutter button--and thus reverting back to AF. Does anyone else use this method??