From what I could find scythed chariots were used in war and were not used by the Romans. The chariots used by racers were Roman racing chariots "were designed to be as small and lightweight as possible. Unlike military chariots, which were larger and often reinforced with metal, racing chariots were made of wood and afforded little support or protection for the charioteer, who basically had to balance himself on the axle as he drove"(http://www.vroma.org/~bmcmanus/circus.html).

Also, I can't really see a spinning blade at the center of a wheel being that effective. If they thought to use a drill bit maybe? But as the quote said, the charioteer was basically balancing on a thin rod. To risk ramming another chariot seems like an pretty foolish move.