It is much politer to be "agnostic" than to be "atheist" - it comes across as much more tolerant - dare one say more enlightened?
However, let's apply a little logic. If you are agnostic, then what you are saying in effect is as follows. There may be a god, or there may not. But if there is a god, then the evidence for his/her/its existence is so slight that you can't definitely conclude that the god is there. God has no effect on the universe.
What is the functional difference between this position and saying "there is no God"?