Sorry but i've got to side with Mark.
If there is no movement at all in the stem then things are just going to snap if you bail, the movement thats their even if its not alot, could be enough to save your bars/ stem.
Many of the companies have doubtlessley thought of this and dismissed it, the machining costs would also increase the price of forks and stems.
You'd be either loosing weight and strength from the cutouts or gaining weight to maintain the same strength.
This is why things are good as they are, threaded headsets went away for moving to much, splined ones obviously haven't turned up because there is no movement, threadless stems are hopefully here for a while.