Daggers are great!
Never underestimate the usefulness of daggers. You don’t use them often, but when you need them there is no substitute.
On the rare occasions that I actually get to play my character I carry a rapier, a shield and my two daggers Spit and Spite.
Spit is carried visibly on the belt and is useful in the second hand if I ever get caught without my shield. i.e. when I am looting treasure or resting.
Spite is my hidden dagger. This is in case I am captured or have to surrender my visible weapons. Also it means that I can appear unarmed and still stab somebody before they can draw.
In combination they are used where the fighting is close range, ie indoors. They are extremely useful against a single long weapon such as a two handed sword or a spear. In our system we generally do not allow one on one combat between people, each with brace of daggers as the combat is so fast that no one can score it.
In Live Role Play daggers are light and are one of the few things that can easily be carried in scabbards. Depending on the system, they can be poisoned, thrown and used to assassinate. They can be held in hand and still be concealed. When you need a dagger there are few substitutes.
Added 27th August:
Also, most game systems do not allow you to fight bare handed. If you do not have a dagger, then there is no way that you can possibly defend yourself in hand to hand combat. You are totally, utterly, completely defenceless and at the mercy of your opponent. As an adventurer would you prefer this, or a dagger so that at least you can attempt to defend yourself?
My advice, always carry a couple on your person and more if you play a thief type character.