Think of it this way. How could you block a blade of plasma? It would slice clean through your arm and still kill you. The only way i see for a sword block to be fair is if you are holding a large weapon, such as a sniper rifle, spartan laser, rocket, plasma launcher, etc. Logically, you could use something like this to soften a sword blow, whereas small arms like a assault rife or DMR would likewise be "sliced up".