Well, bayonets no longer serve their purpose because enemies gain little advantage by getting close in real life. Most likely you both have guns and they both are probably extremely acurate from a hundred or so feet away. At the time when bayonets were in common use soldiers could charge the battlefield between reloading (which could take a while) and engage in close combat. A useful addition was the blade on the end of the weapon to fight when they approached. A Marine using a bayonet could more easily repel a zergling which can get in close very quickly.
Post Script: After searching I have found that bayonts are still used for close combat in modern times, though not as commonly.
Post Script: After searching I have found that bayonts are still used for close combat in modern times, though not as commonly.