Almost all infantry soldiers IRL have some sort of daggers as backup weapon. Your main weapon can be broken, you can be wounded, therefore cant use both your hand with polearm weapon, in some crowd situation you even cant raise your hand, so dagger is only way to defend yourself.
And you definitely CAN block with cinquedea, or flamed dagger, or other daggers with blade length 40+, solid guard and 0.5+ weight.
About our daggers weight - sorry, mine Leatherman multitool weigh more than ingame dagger. And this is just silly.