There is no such thing as agnosticism, at least as a different stance of belief from atheism. To say that agnostics accept the possibility of a god, where an atheist does not is simply wrong. I don't think anyone would make that claim, but saying that agnosticism means accepting the impossibility for definitive proof either way, therefore making judgments on the likelihood of gods useless, is not logically sound.
Making a leap from unprovable to untestable is completely unfounded. The question of labels comes down to the onus of evidence. To claim the existence of any particular god or system of metaphysics, you require empirical evidence and logical argument of its truth. The burden of proof is on whom ever makes the claim , and the evidence must be testable.
The difference between strong and weak atheism, I believe, is also a false division. To say strong atheists believe positivly that there is no god, and weak atheists simply don't have a belief, is to say nothing of meaning. The only reasoning that could possibly hold is that weak atheists either haven't looked into the issue sufficiently to make a judgment, or believe that every claim to a gods existence has sufficient evidence to be just as likely as its non existence.
The last apparent possibility to distinguish a strong atheist is one who claims to be sure there are no gods. This would be a wholly irrational claim, and doesn't deserve the dignity of an official label.