Just a couple points : Spear/shield isn't a dueling setup, but that doesn't mean that you can't 1v1 effectively. Really fast people will kill you, since the animation for S/S is a little slow, but you can learn to take down an average player pretty efficiently.
The problem with having an overhead attack with S/S, is what would the animation look like? If it looks anything like a normal thrust, then it'd be way too difficult to block, which would make it over powered.
Also, only the Huscarl shield and Board Shields are going to take double slots. Kite shield, Cav shield, Heater Shield, etc. will still allow you to bring a back up 1h to use if you get into an uncomfortable spot.