Seems to me agnosticism is the default position.
If you don't know saying you don't know seems to make more sense than either defaulting to yes or no.
Agnoticism does not address the question at hand.
The question is if you believe a God exists, not if you know a God exists.
If you hold a positive belief that a God exists, then you are a Theist. If you do not hold a positive belief that God exists, then you are an Atheist.
It's a binary proposition, you either have a belief, or you are without belief.
Agnosticism (or gnosticism) deals with what you know. If you claim to know a God does or does not exist, you are Gnostic. If you claim to not know for sure whether a God exists, then you are agnostic.
Therefore you can be a:
Gnostic Theist: You believe in God's existence, and claim to know for sure he exists
Agnostic Theist: You believe in God's existence, but do not claim to know for sure
Agnostic Atheist: You do not believe a God exists, but you don't claim to know for certain
Gnostic Atheist: You do not believe a God exists, and you claim to know that no Gods exist.
Most Atheists fall under the Agnostic category, as you can not disprove the existence of a God.
However, that is why Atheism is the default position. It is irrational to hold a belief in something when you do not have evidence to support that belief.