Aren't beliefs secondary guests in discussions about substance?

Edit: I'm not even a strict atheist if that matters much, but I wouldn't talk about it in assertive/provemewrong tone anywhere, cause it's not even remotely logical.

Atheism is such a weird position to take. Why define a negation of something you don't believe to be real? I'm not "a-lochnes-monster-ist" or "ayettist". If someone thinks the idea of a deity is nonsensical why then use the label "atheist"?