Agnosticism is being used as a special case with regards to atheism in order to correct the (deliberate) misrepresentation of atheism by theists in order to gain a level playing field, that most really know to be false.. You know the stuff - the "your disbelief is a belief" malarckey, which conveniently only seems to be applicable to atheism and no other disbeliefs. That's how this agnostic label really took off the ground, when, as Grendel has said, it isn't needed.
On the other foot, while the use of the label seems to be an obvious default for all, it's still correct to use it if you are agnostic. It's not like it's being used incorrectly (by those who know what it means in each given context). To those of us who do see it as obvious, we just reply, "Well, duh!", but then others, such as theists, just don't get the difference (sometimes deliberately) between not believing in the existence of gods and believing that no gods exist.
I suppose, a bit like claiming your an atheist, the idea of having to say your an agnostic shouldn't be required. To me, the atheist "fight" is there to negate its own purpose, so perhaps that can be extended to agnosticism.... with regards to atheism at least.