Spear&Shield combo will not work without the speed. Quick thrusts, m8. Quick thrusts. That's the only way to fight with Spear&Shield in M&B unless they implement some kind of new mechanic, but I don't see that happening. IRL, tho, the S&S could do so much more... =/
fighting AI spearmen with the highest combat AI setting is quite a thing, they can effectively parry/chamber EVERY swing or thrust you do and flawlessly get a stab in after - they don't even need a shield. Such things were certainlly not possible with spears in native
Spears used to swing or to block should break easily, they weren't really made for that. Trully, shield and spear would require fast attack, manly due to how block works in the game and the shield limiting spear use to a single attack direction, but if they began braking trough improper use, it could change the scenario.