When Spartacus first meets Diotimos, he engages in conversation with his sword in hand. Amid asking Diotimos if he "expected freedom to come absent cost," he sheathes his sword. However, in the immediate following shot, we see him again sheathing his sword after already having done so.
Although Roman Legionaries of this period carried a spear (more accurately a javelin) called a pilum, it was a throwing weapon not a stabbing one. Legionaries trained well with them and could throw them hard and accurately enough to pierce an opponent's shield (or an opponent if without a shield) A frontal assault by a single rider would, in reality, only end one way.