I was recently involved in an online discussion in which a reason I hadn't previously seen was offered for preferring negative to positive atheism. (By negative atheism, I mean the mere lack of belief in any gods, and by positive atheism, the belief that there are no gods. And the fact that one usually needs to explain this is one reason I prefer the traditional terminology.)
There are better and worse reasons for being only a negative atheist. But the one that was argued by my opponent in the discussion was pretty weak — and if it is accepted by others who call themselves atheists, they really should be aware of that.
Briefly, my opponent's argument was that one should only believe when there is evidence; that there is no evidence that there are no gods; and therefore that to positively disbelieve in such beings is completely unjustified.