In reply to iksander:
yeah, but even a straight shafted tool will be less stable on the upper griprest as a smaller amount of movement at a higher point on the shaft is needed to move the pick the same amount, a left to right 'wiggle' of a few degrees when you're on the upper grip position is equivalent to a much bigger wriggle on the lower handle as it moves in a tighter arc, this is partly why all tools feel 'twitchier' choked up.
Grivel's Quantum Monster competition tools tried to reduce the difference in 'feel' between the two positions by overlapping them, making the upper grip position closer to the wall and a little lower but as you say this compromises clearance and the number of ways the axe can be used (daggering, plunging etc) so is unlikely to be adopted in 'proper' (ie. for climbing real ice / snow / rock rather than competition DT / ice) axe design.