Yes, it is necessary. Otherwise, there would be no reason for demographers to distinguish between these two groups, since that makes literally no sense if they believe exactly the same thing.
Check out this definition:
Atheism is, in the broadest sense, the absence of belief in the existence of deities. Less broadly, atheism is the rejection of belief that any deities exist. In an even narrower sense, atheism is specifically the position that there are no deities. Atheism is contrasted with theism, which, in its most general form, is the belief that at least one deity exists.
(
https://en.wikipedia.org/wiki/Atheism)
The important part is that Atheism, no mater how broad the definition, is contrasted with theism. This is pretty obvious from the name, and this is why it is a different idea from agnosticism.
Edited 6/5/2016 02:43:16
